make <mapper> documentation easier to read.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@269903 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTypes/mapper.html

@@ -67,10 +67,11 @@ with the following attributes:</p>
 the <code>to</code> and <code>from</code> attributes to the correct
 directory separator of your current platform.  If you need to specify
 this separator, use <code>${file.separator}</code> instead.</p>
+<h3>Parameters specified as nested elements</h3>
 <p>The classpath can be specified via a nested
 <code>&lt;classpath&gt;</code>, as well - that is,
 a <a href="../using.html#path">path</a>-like structure.</p>
-<p>The built-in mapper types are:</p>
+<h3>The built-in mapper types are:</h3>
 <h4><a name="identity-mapper">identity</a></h4>
 <p>The target file name is identical to the source file name. Both
 <code>to</code> and <code>from</code> will be ignored.</p>