docs: Updated CONTRIBUTING.md (#479)

Added missing comma on line 18.

Author: nathanlschneider

CONTRIBUTING.md

@@ -15,7 +15,7 @@ Our open source community strives to be nice, welcoming and professional. Instan
 * Before submitting a bug report, search the issues for similar tickets. Your issue may have already been discussed and resolved. 
 * Feel free to add a comment to an existing issue, even if it's closed.
 * Be thorough in your title and report, don't leave out important details.
-* English please.
+* English, please.
 
 ## Tests