doc: submodule.* config are keyed by submodule names

Ever since 941987a5 (git-submodule: give submodules proper names,
2007-06-11) introduced the ability to move a submodule from one path
to another inside its superproject tree without losing its identity,
we should have consistently used submodule.<name>.* to access
settings related to the named submodule.
Reported-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NW. Trevor King <wking@tremily.us>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -208,7 +208,7 @@ OPTIONS
 -b::
 --branch::
 	Branch of repository to add as submodule.
-	The name of the branch is recorded as `submodule.<path>.branch` in
+	The name of the branch is recorded as `submodule.<name>.branch` in
 	`.gitmodules` for `update --remote`.
 
 -f::