doc: submodule: fix typo for command absorbgitdirs

The sentence wants to talk about the superproject's possesive, not plural form.

Signed-off-by: Thomas Menzel <dev@tomsit.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -226,7 +226,7 @@ registered submodules, and sync any nested submodules within.
 
 absorbgitdirs::
 	If a git directory of a submodule is inside the submodule,
-	move the git directory of the submodule into its superprojects
+	move the git directory of the submodule into its superproject's
 	`$GIT_DIR/modules` path and then connect the git directory and
 	its working directory by setting the `core.worktree` and adding
 	a .git file pointing to the git directory embedded in the