gitmodules.txt: fix 'GIT_WORK_TREE' variable name

gitmodules.txt: fix 'GIT_WORK_TREE' variable name

'gitmodules.txt' is a guide about the '.gitmodules' file that describes
submodule properties, and that file must exist at the root of the
repository. This was clarified in e5b5c1d2cf (Document clarification:
gitmodules, gitattributes, 2008-08-31).

However, that commit mistakenly uses the non-existing environment
variable 'GIT_WORK_DIR' to refer to the root of the repository.

Fix that by using the correct variable, 'GIT_WORK_TREE'. Take the
opportunity to modernize and improve the formatting of that guide,
and fix a grammar mistake.

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