Fix typo
update CONTRIBUTING.md

Tripped across this; not sure if other pages need to be changed.
+ http://csscomb.com/config tip
Merge pull request #324 from tomByrer/patch-1

update CONTRIBUTING.md
Merge pull request #325 from tomByrer/patch-2

+ http://csscomb.com/config tip
Merge pull request #318 from gonzalocasas/patch-1

Fix typo