READMEs: CHANGE callback replaces SET/CLEAR_MODE
READMEs: CHANGE callback replaces SET/CLEAR_MODE

LWS_CALLBACK_SET_MODE_POLL_FD and LWS_CALLBACK_CLEAR_MODE_POLL_FD no longer exist.

Updating the documentation to reflect that and point to LWS_CALLBACK_CHANGE_MODE_POLL_FD.

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