Fix typo in color managed workflow docs

Merged Jay Kamat requested to merge jgkamat/docs-krita-org:jay/fix-18bit-typo into master

I'm not sure if 18 bit color exists, but it makes sense (in my mind at least) that banding would only occur when reducing the color depth, not increasing it. Since 8 bit was referenced above, I think that was the intention.

That said, if this is incorrect, please just close this :)

Edited by Jay Kamat

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