Fix ExceptionUtils#getStackFrame(String) javadoc (closes #274)

ExceptionUtils#getStackFrame(String)'s javadoc contains a broken
reference to SystemUtils#LINE_SEPARATOR (as the class name is not
fully qualified, and there's no import of
org.apache.commons.lang3.SystemUtils).

This patch fixes the broken reference by replacing it it with a
reference to System#lineSeparator(), which the method actually uses.

src/main/java/org/apache/commons/lang3/exception/ExceptionUtils.java

@@ -605,7 +605,7 @@ public static String[] getStackFrames(final Throwable throwable) {
     /**
      * <p>Returns an array where each element is a line from the argument.</p>
      *
-     * <p>The end of line is determined by the value of {@link SystemUtils#LINE_SEPARATOR}.</p>
+     * <p>The end of line is determined by the value of {@link System#lineSeparator()}.</p>
      *
      * @param stackTrace  a stack trace String
      * @return an array where each element is a line from the argument