qapi/migration: Fix examples document wrong field name for arguments

qapi/migration: Fix examples document wrong field name for arguments

The examples for the snapshot-* and calc-dirty-rate commands document
that arguments for the commands are passed in a 'data' field.
This is wrong, passing them in a "data" field results in
the error:
  {"error": {"class": "GenericError", "desc": "QMP input member 'data'
		       is unexpected"}}
Arguments are expected to be passed in an field called "arguments".

Replace "data" with "arguments" in the snapshot-* and calc-dirty-rate
command examples.

Signed-off-by: Fabian Holler <fabian.holler@simplesurance.de>
Message-Id: <20220222170116.63105-1-fabian.holler@simplesurance.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>