MINOR: Use new-consumer config in MirrorMaker doc
MINOR: Use new-consumer config in MirrorMaker doc

Mirrormaker was updated to default to the new consumer in 3db752a565071c78e4b11eaafa739844fa785b04

Old consumer calls the param `auto.commit.enable`, new consumer calls it `enable.auto.commit`. So I updated the docstring to use the new consumer name for the param.

Author: Jeff Widman <jeff@jeffwidman.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>

Closes #2393 from jeffwidman/patch-1

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