fix syntax highlight for git user

It doesn't look right to me. PR welcome if you think otherwise :)

README.md

@@ -1399,7 +1399,7 @@ Options:
 
 ## [git-user](https://github.com/gesquive/git-user)
 
-```shell
+```
 # add a work profile for Henry
 $ git user add work "Dr. Henry Jekyll" henry@jekyll.com
 Added profile 'work'