gitsubmodules doc: invoke 'ls-files' with '--recurse-submodules'

gitsubmodules doc: invoke 'ls-files' with '--recurse-submodules'

`git ls-files` was never taught to respect the `submodule.recurse`
configuration variable, and it is too late now to change that [1],
but still the command is mentioned in 'gitsubmodules(7)' as if it
does respect that config.

Adjust the call in 'gitsubmodules(7)' by calling 'ls-files' with the
'--recurse-submodules' option.

While at it, uniformize the capitalization in that file, and use
backticks instead of quotes for Git commands and configuration
variables.

[1] https://lore.kernel.org/git/pull.732.git.1599707259907.gitgitgadget@gmail.com/T/#u

Signed-off-by: Philippe Blain <redacted>
Signed-off-by: Junio C Hamano <redacted>