Update programming-guide w/Java GroupByKey example

Minor addition to assist new users with a GroupByKey example in Java. Specifically:

Add the following content to provide a Java example at the end of section 4.2.2. GroupByKey.

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Using GroupByKey is straightforward:

< highlight java >
// The input PCollection.
PCollection<KV<String, String>> mapped = ...;

// Apply GroupByKey to the PCollection mapped.
// Save the result as the PCollection reduced.
PCollection<KV<String, Iterable>> reduced =
mapped.apply(GroupByKey.<String, String>create());

}