Update CONTRIBUTING.md

Grammatical correctness.
Merge pull request #1587 from jwyterlin/patch-1

Update CONTRIBUTING.md
Remove enclosing brackets around gitlab push notifiers

The presence of the brackets is inconsistent with the rest of the
messages, and in some services it causes the residing url to break (as
the closing parenthesis is seen as part of the url)
Merge pull request #1589 from FriedSock/master

Remove enclosing brackets around gitlab push notifiers