Fix javadoc issues

src/main/java/org/apache/commons/math4/stat/descriptive/moment/GeometricMean.java

@@ -35,7 +35,7 @@ import org.apache.commons.math4.util.MathUtils;
  * Uses a {@link SumOfLogs} instance to compute sum of logs and returns
  * <code> exp( 1/n  (sum of logs) ).</code>  Therefore, </p>
  * <ul>
- * <li>If any of values are < 0, the result is <code>NaN.</code></li>
+ * <li>If any of values are {@code < 0}, the result is <code>NaN.</code></li>
  * <li>If all values are non-negative and less than
  * <code>Double.POSITIVE_INFINITY</code>,  but at least one value is 0, the
  * result is <code>0.</code></li>

src/main/java/org/apache/commons/math4/stat/descriptive/moment/Kurtosis.java

@@ -36,7 +36,7 @@ import org.apache.commons.math4.util.MathUtils;
  * where n is the number of values, mean is the {@link Mean} and std is the
  * {@link StandardDeviation}</p>
  * <p>
- * Note that this statistic is undefined for n < 4.  <code>Double.Nan</code>
+ * Note that this statistic is undefined for {@code n < 4}.  <code>Double.Nan</code>
  * is returned when there is not sufficient data to compute the statistic.
  * Note that Double.NaN may also be returned if the input includes NaN
  * and / or infinite values.</p>

src/main/java/org/apache/commons/math4/stat/descriptive/moment/SemiVariance.java

@@ -30,7 +30,7 @@ import org.apache.commons.math4.util.MathUtils;
  * We define the <i>downside semivariance</i> of a set of values <code>x</code>
  * against the <i>cutoff value</i> <code>cutoff</code> to be <br>
  * <code>&Sigma; (x[i] - target)<sup>2</sup> / df</code> <br>
- * where the sum is taken over all <code>i</code> such that <code>x[i] < cutoff</code>
+ * where the sum is taken over all <code>i</code> such that {@code x[i] < cutoff}
  * and <code>df</code> is the length of <code>x</code> (non-bias-corrected) or
  * one less than this number (bias corrected).  The <i>upside semivariance</i>
  * is defined similarly, with the sum taken over values of <code>x</code> that

src/main/java/org/apache/commons/math4/stat/descriptive/moment/Skewness.java

@@ -35,7 +35,7 @@ import org.apache.commons.math4.util.MathUtils;
  * where n is the number of values, mean is the {@link Mean} and std is the
  * {@link StandardDeviation} </p>
  * <p>
- * Note that this statistic is undefined for n < 3.  <code>Double.Nan</code>
+ * Note that this statistic is undefined for {@code n < 3}.  <code>Double.Nan</code>
  * is returned when there is not sufficient data to compute the statistic.
  * Double.NaN may also be returned if the input includes NaN and / or
  * infinite values.</p>

src/main/java/org/apache/commons/math4/stat/descriptive/moment/Variance.java

@@ -276,10 +276,10 @@ public class Variance extends AbstractStorelessUnivariateStatistic implements Se
      * the input array, or <code>Double.NaN</code> if the designated subarray
      * is empty.</p>
      * <p>
-     * Uses the formula <pre>
+     * Uses the formula <div style="white-space: pre"><code>
      *   &Sigma;(weights[i]*(values[i] - weightedMean)<sup>2</sup>)/(&Sigma;(weights[i]) - 1)
-     * </pre>
-     * where weightedMean is the weighted mean</p>
+     * </code></div>
+     * where weightedMean is the weighted mean
      * <p>
      * This formula will not return the same result as the unweighted variance when all
      * weights are equal, unless all weights are equal to 1. The formula assumes that