submodule foreach: replace $path with $sm_path in example

f0fd0dc5c5 (submodule foreach: document '$sm_path' instead of '$path',
2018-05-08) updated the documentation to advise callers to favor
$sm_path over the deprecated synonym $path.  However, the example in
that section still uses $path.  Update it to use $sm_path.

Signed-off-by: Kyle Meyer <kyle@kyleam.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -219,7 +219,7 @@ As an example, the command below will show the path and currently
 checked out commit for each submodule:
 +
 --------------
-git submodule foreach 'echo $path `git rev-parse HEAD`'
+git submodule foreach 'echo $sm_path `git rev-parse HEAD`'
 --------------
 
 sync [--recursive] [--] [<path>...]::