MINOR: Rename description of flatMapValues transformation (#482)

The table of (stateless) transformations uses the transformation name in the first column and a description in the second column. I adjusted the transformation name for FlatMapValues accordingly.

See also Kafka #8431
Reviewers: Bill Bejeck <bbejeck@apache.org>

33/streams/developer-guide/dsl-api.html

@@ -463,7 +463,7 @@ <h4 class="anchor-heading"><a id="streams_concepts_globalktable" class="anchor-l
 // Java 7 example: cf. `map` for how to create `KeyValueMapper` instances</code></pre>
                         </td>
                     </tr>
-                    <tr class="row-even"><td><p class="first"><strong>FlatMap (values only)</strong></p>
+                    <tr class="row-even"><td><p class="first"><strong>FlatMapValues</strong></p>
                         <ul class="last simple">
                             <li>KStream &rarr; KStream</li>
                         </ul>