CONTRIBUTING: fix code block formatting

There were only 3 spaces instead of 4, so the example was being considered to include html elements

CONTRIBUTING.md

@@ -51,7 +51,7 @@ see if the change were accepted.
 
 The title should look like:
 
-   <package>: <short description>
+    <package>: <short description>
 
 The package is the most affected Go package. If the change does not affect Go
 code, then use the directory name instead. Changes to a single well-known