p7821: fix instructions for testing with threads

p7821: fix instructions for testing with threads

In 7b31b55db1 (perf: amend the grep tests to test grep.threads,
2017-12-29), p7821 was tweaked to test the performance of 'git grep'
under different number of threads. These tests are run if
GIT_PERF_GREP_THREADS is set to a list of thread numbers, but the
comment at the top of the file instead mentions GIT_PERF_7821_THREADS.
Fix the comment.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>