CommonMistakes: change title

The x/website tests look for the space.

Change-Id: I9dfa025acac6f7f44d03fd9743a485641ab4a3d6
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/550155
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: rsc

CommonMistakes.md

@@ -1,5 +1,5 @@
 ---
-title: CommonMistakes
+title: Common Mistakes
 ---
 
 Table of Contents