Correct command example in README.md

[lingkai5wu]

Fixed command syntax and parameters in README.

[flwyd]

Thanks for contributing to ADIF Multitool, and sorry for the delayed review.

The typo correction is obviously correct. Could you say why you propose removing pota_ref from the flatten example? Parks on the Air currently expects 2-fers to be logged as two separate contacts, and flattening the pota_ref field allows adifmt infer to set the sig_info field. (Though I neglected to include sig,sig_info,my_sig,my_sig_info in the infer command right after.)