[libFuzzer] Fixed description of fuzzer merge control file.
The description of the fuzzer merge control file appears to be incorrect/out of date.
No "DONE" line appears in the control file. Rather, FT and COV are the markers that appear
following the STARTED line.
Reviewers: metzman, kcc
Reviewed By: kcc
Differential Revision: https://reviews.llvm.org/D80788