git-submodule: use the standard directory convention

pages/common/git-submodule.md

@@ -12,7 +12,7 @@
 
 - Add a git repository as a submodule at the specified directory:
 
-`git submodule add {{repository_url}} {{directory}}`
+`git submodule add {{repository_url}} {{path/to/directory}}`
 
 - Update every submodule to its latest commit: