help: add gitsubmodules to the list of guides

The guide "gitsubmodules" was added in d480345 (submodules: overhaul
documentation, 2017-06-22), but it was not added to
command-list.txt when commit 1b81d8c (help: use command-list.txt
for the source of guides, 2018-05-20) taught "git help" to obtain the
guide list from this file.

Add it now, and capitalize the first word of the description of
gitsubmodules, as was done in 1b81d8c (help: use command-list.txt
for the source of guides, 2018-05-20) for the other guides.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitsubmodules.txt

@@ -3,7 +3,7 @@ gitsubmodules(7)
 
 NAME
 ----
-gitsubmodules - mounting one repository inside another
+gitsubmodules - Mounting one repository inside another
 
 SYNOPSIS
 --------

command-list.txt

@@ -203,6 +203,7 @@ gitmodules                              guide
 gitnamespaces                           guide
 gitrepository-layout                    guide
 gitrevisions                            guide
+gitsubmodules                           guide
 gittutorial-2                           guide
 gittutorial                             guide
 gitworkflows                            guide