[Docs] minor grammar fix

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #2430 from nchammas/patch-2 and squashes the following commits:

d476bfb [Nicholas Chammas] [Docs] minor grammar fix

CONTRIBUTING.md

@@ -8,5 +8,5 @@ submitting any copyrighted material via pull request, email, or other means
 you agree to license the material under the project's open source license and
 warrant that you have the legal authority to do so.
 
-Please see [Contributing to Spark wiki page](https://cwiki.apache.org/SPARK/Contributing+to+Spark)
+Please see the [Contributing to Spark wiki page](https://cwiki.apache.org/SPARK/Contributing+to+Spark)
 for more information.