CONTRIBUTING.md: punctuation fixes

Signed-off-by: Marc Abramowitz <marc@marc-abramowitz.com>

CONTRIBUTING.md

@@ -1,7 +1,7 @@
 # Contributing to Docker
 
 Want to hack on Docker? Awesome! Here are instructions to get you
-started. They are probably not perfect, please let us know if anything
+started. They are probably not perfect; please let us know if anything
 feels wrong or incomplete.
 
 ## Topics