gitmodules.5: url can be a relative path

gitmodules.5: url can be a relative path

There is already excellent documentation for this facility in
git-submodule.1, but it is not so discoverable.

Relative paths in .gitmodules can be useful for serving the
same repository over multiple protocols, for example.
Thanks to Peter for pointing this out.

Cc: Peter Krefting <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Acked-by: Johan Herland <redacted>
Signed-off-by: Junio C Hamano <redacted>