add a missing 'a'

added a missing 'a' in line 72.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f37c5cd..71e2e0d 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -69,7 +69,7 @@
 all of open source.
 
 As always, discuss Google Test in the official GoogleTest discussion group. You
-don't have to actually submit code in order to sign up. Your participation
+don't have to actually submit a code in order to sign up. Your participation
 itself is a valuable contribution.
 
 ## Style