doc: clarify which remotes can be used with GitGitGadget

doc: clarify which remotes can be used with GitGitGadget

The docs mostly point to using git/git as one's remote, however, when it
comes to Sending a PR to GitGitGadget section, the reader is told to use
gitgitgadget/git, with no mention of git/git, potentially leading to
some confusion.

Clarify that both gitgitgadget/git and git/git can be used, albeit with
some differences.

Signed-off-by: Daniele Sassoli <danielesassoli@gmail.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>