Documentation/git-submodule.txt: typofix

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -67,7 +67,7 @@ FILES
 -----
 When initializing submodules, a .gitmodules file in the top-level directory
 of the containing repository is used to find the url of each submodule.
-This file should be formatted in the same way as $GIR_DIR/config. The key
+This file should be formatted in the same way as `$GIT_DIR/config`. The key
 to each submodule url is "submodule.$name.url".  See gitlink:gitmodules[5]
 for details.