MINOR: MyProcessor doc example should implement, not extend `Processor`
Author: Michael G. Noll
Reviewers: Eno Thereska, Guozhang Wang
Closes from miguno/trunk-streams-docs-typo-fix
</p>
<pre>
-public class MyProcessor extends Processor&lt;String, String&gt; {
+public class MyProcessor implements Processor&lt;String, String&gt; {
private ProcessorContext context;
private KeyValueStore&lt;String, Long&gt; kvStore;
