Manifest: fix javadoc of method item() (#575)

Method `getSize` doesn't exist, so replace a reference to it in
a documentation comment of method `item()` with `getLength`.
Fix the Javadoc syntax of `{@link ...}`, while we're here.

Originally noticed in the code imported into the openjdk repository:
https://github.com/openjdk/jdk/blob/master/src/java.xml.crypto/share/classes/com/sun/org/apache/xml/internal/security/signature/Manifest.java#L228-L229

Author: rybak

src/main/java/org/apache/xml/security/signature/Manifest.java

@@ -221,7 +221,7 @@ public int getLength() {
 
     /**
      * Return the <i>i</i><sup>th</sup> reference. Valid <code>i</code>
-     * values are 0 to <code>{link@ getSize}-1</code>.
+     * values are 0 to <code>{@link #getLength()}-1</code>.
      *
      * @param i Index of the requested {@link Reference}
      * @return the <i>i</i><sup>th</sup> reference