doc: Correct usage of `kas shell` in documentation

The correct syntax for `kas shell` is
    `kas shell [-c COMMAND]`
or
    `kas shell [--command COMMAND]`
rather than
    `kas shell [-c=COMMAND]`

Anything else may work with a given kas version (by design or by
accident) but is not documented behavior

Signed-off-by: Adam Johnston <adam.johnston@arm.com>