Updated programming-guide.md

Made sure that readers know the random number generator seed argument, within the 'takeSample' method, is optional.

Author: Rishi Verma <[email protected]>

Closes #1324 from riverma/patch-1 and squashes the following commits:

4699676 [Rishi Verma] Updated programming-guide.md

Author: riverma

docs/programming-guide.md

@@ -975,8 +975,8 @@ for details.
   <td> Return an array with the first <i>n</i> elements of the dataset. Note that this is currently not executed in parallel. Instead, the driver program computes all the elements. </td>
 </tr>
 <tr>
-  <td> <b>takeSample</b>(<i>withReplacement</i>, <i>num</i>, <i>seed</i>) </td>
-  <td> Return an array with a random sample of <i>num</i> elements of the dataset, with or without replacement, using the given random number generator seed. </td>
+  <td> <b>takeSample</b>(<i>withReplacement</i>, <i>num</i>, [<i>seed</i>]) </td>
+  <td> Return an array with a random sample of <i>num</i> elements of the dataset, with or without replacement, optionally pre-specifying a random number generator seed.</td>
 </tr>
 <tr>
   <td> <b>takeOrdered</b>(<i>n</i>, <i>[ordering]</i>) </td>