LANG-1314: Fix javadoc creation on with Java 8

Remove </p> tag from Computable's javadoc

The standard javadoc doclet does not allow self closing tags (such as
</p>). This patch removes such a tag from Computable's javadoc, as it's
redundant anyway, as it's only used to create spaces between two
existing paragraphs.

src/main/java/org/apache/commons/lang3/concurrent/Computable.java

@@ -18,7 +18,7 @@ package org.apache.commons.lang3.concurrent;
 
 /**
  * <p>Definition of an interface for a wrapper around a calculation that takes a single parameter and returns a result.</p>
- * <p/>
+ *
  * <p>This interface allows for wrapping a calculation into a class so that it maybe passed around an application.</p>
  *
  * @param <I> the type of the input to the calculation