adding example usage for command option

[TZubiri]

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Hello, please take a look at this small documentation change.

Basically I struggled for a while to figure out why the monitor wasn't working, and I had to play with the IDE to understand that I needed to match the baud rates.

I'm adding a really low effort change that might help users work out how to use the -c option once they figured out they need to change the baud rate.

Ideally I think the options should be collapsed into command line flags so that the following works:

arduino-cli monitor --baudrate 1000000

But for now I think this small change can be pulled, if it can be tested by someone with a working build.

If possible, I would also change the flag usage from

-c, --config strings

to

-c --config ID=Setting

in addition to clarifying the syntax, it also points the user towards the --describe keywords.

Thank you. Tom

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