Clarify request-branch 'service levels' argument
Description of 'request-branch' command wasn't clear for '--sl' argument
and was modified. '--sl' argument allows multiple values and therefore
only positional argument 'branch' shouldn't be placed at the end of
line.
JIRA: COMPOSE-2970
Fixes: #283
Signed-off-by: Ondrej Nosek <onosek@redhat.com>