feat: adds help text to --log

[EverlastingBugstopper]

just realized we didn't have any help text for this flag!

before:

$ rover --help
...
OPTIONS:
    -l, --log <log-level>     [possible values: error, warn, info, debug, trace]
...

after:

$ rover --help
...
OPTIONS:
    -l, --log <log-level>    Specify Rover's log level [possible values: error, warn, info,
                             debug, trace]
...