[DOCS] Fix typo in return type of cogroup

This fixes a simple typo in the cogroup docs noted in http://mail-archives.apache.org/mod_mbox/spark-user/201501.mbox/%3CCAMAsSdJ8_24evMAMg7fOZCQjwimisbYWa9v8BN6Rc3JCauja6wmail.gmail.com%3E

I didn't bother with a JIRA

Author: Sean Owen <[email protected]>

Closes #4072 from srowen/CogroupDocFix and squashes the following commits:

43c850b [Sean Owen] Fix typo in return type of cogroup

Author: srowen

docs/programming-guide.md

@@ -913,7 +913,7 @@ for details.
 </tr>
 <tr>
   <td> <b>cogroup</b>(<i>otherDataset</i>, [<i>numTasks</i>]) </td>
-  <td> When called on datasets of type (K, V) and (K, W), returns a dataset of (K, Iterable&lt;V&gt;, Iterable&lt;W&gt;) tuples. This operation is also called <code>groupWith</code>. </td>
+  <td> When called on datasets of type (K, V) and (K, W), returns a dataset of (K, (Iterable&lt;V&gt;, Iterable&lt;W&gt;)) tuples. This operation is also called <code>groupWith</code>. </td>
 </tr>
 <tr>
   <td> <b>cartesian</b>(<i>otherDataset</i>) </td>