git-submodule.txt: fix AsciiDoc formatting error

git-submodule.txt: fix AsciiDoc formatting error

In b57e8119e6 (submodule: teach set-branch subcommand, 2019-02-08), the
`set-branch` subcommand was added for submodules. When the documentation
was written, the syntax for a "index term" in AsciiDoc was
accidentally used. This caused the documentation to be rendered as

	set-branch -d|--default)|(-b|--branch <branch> [--] <path>

instead of

	set-branch ((-d|--default)|(-b|--branch <branch>)) [--] <path>

In addition to this, the original documentation was possibly confusing
as it made it seem as if the `-b` option didn't accept a `<branch>`
argument.

Break `--default` and `--branch` into their own separate invocations to
make it obvious that these options are mutually exclusive. Also, this
removes the surrounding parentheses so that the "index term" syntax is
not triggered.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>