Fix trivial typo

CONTRIBUTING.md

@@ -47,7 +47,7 @@ showing just the first line of each commit message, usually prefixed
 by some metadata on the left.  (And the reason for leaving the period
 off the subject line is thus to save space.)
 
-Think of the commit message is an introduction to the change.  A
+Think of the commit message as an introduction to the change.  A
 reviewer will read the commit message right before reading the diff
 itself, so the commit message's purpose is to put the reader in the
 right frame of mind to understand the code change.  The level of