p7821: fix instructions for testing with threads

In 7b31b55 (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>

Author: phil-blain

t/perf/p7821-grep-engines-fixed.sh

@@ -7,7 +7,7 @@ git-grep. Make sure to include a leading space,
 e.g. GIT_PERF_7821_GREP_OPTS=' -w'. See p7820-grep-engines.sh for more
 options to try.
 
-If GIT_PERF_7821_THREADS is set to a list of threads (e.g. '1 4 8'
+If GIT_PERF_GREP_THREADS is set to a list of threads (e.g. '1 4 8'
 etc.) we will test the patterns under those numbers of threads.
 "