[libFuzzer] Fixed description of fuzzer merge control file.

Summary:
The description of the fuzzer merge control file appears to be incorrect/out of date.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

Subscribers: #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D80788