docs: standardize positional arg notation in mr commands
Description
Fixes the positional argument notation in three glab mr command
usage lines to conform to the GitLab CLI style guide.
The previous notation used curly braces ({<id> | <branch>}), which
is not defined in the style guide and was inconsistent with all other
mr subcommands. The correct convention for an optional argument with
multiple accepted forms is [<id | branch>].
Related issues
Documentation
If this MR adds or updates CLI command documentation, follow the conventions in the
GitLab CLI (glab) documentation style guide,
run make gen-docs, and commit the updated files in docs/source/.