Fix typo in CONTRIBUTING.rst
Fix typo in CONTRIBUTING.rst

@LindsayHill's eagle eyes spotted a typo that I overlooked in my previous patch:
https://github.com/StackStorm/st2/pull/2984

This fixes that.

Signed-off-by: Matt Oswalt <14ae767ed7efb9f763e9501895dea40bd4bd6ed4@brocade.com>

lakshmi-kannan/st2,peak6/st2,nzlosh/st2,StackStorm/st2,peak6/st2,nzlosh/st2,Plexxi/st2,lakshmi-kannan/st2,Plexxi/st2,tonybaloney/st2,lakshmi-kannan/st2,StackStorm/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,tonybaloney/st2,peak6/st2,nzlosh/st2,Plexxi/st2,tonybaloney/st2,StackStorm/st2