stbt-batch: Fix inconsistency in help message
stbt-batch: Fix inconsistency in help message

The help message of `stbt batch` says that "For help on a specific
subcommand do 'stbt batch <subcommand> --help'". However, running
`stbt batch run --help` exits with status code 1 saying
"Invalid option '--'. Use '-h' for help."

Advise users to use 'stbt batch <subcommand> -h' to avoid this error.

LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester