user-manual.txt: add missing section label

We provide a label for each chapter and section except for the "Pitfalls
with submodules" section. Since we're doing it everywhere else, let's do
it here, too.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -3698,6 +3698,7 @@ $ git push
 You have to run `git submodule update` after `git pull` if you want to update
 submodules, too.
 
+[[pitfalls-with-submodules]]
 Pitfalls with submodules
 ------------------------