Fix typo in doc for combine.
Fix typo in doc for combine.

hugovk/coveragepy,blueyed/coveragepy,nedbat/coveragepy,hugovk/coveragepy,hugovk/coveragepy,hugovk/coveragepy,nedbat/coveragepy,blueyed/coveragepy,nedbat/coveragepy,blueyed/coveragepy,nedbat/coveragepy,blueyed/coveragepy,nedbat/coveragepy,hugovk/coveragepy,blueyed/coveragepy