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>