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>