Correct comment to refer to SkImageFilter::kReverse_MapDirection.

kBackward_MapDirection does not exist; might as well have the comment exactly
match the valid enumerator.

TBR=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1820883002

Review URL: https://codereview.chromium.org/1820883002
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