perf: update documentation of GIT_PERF_REPEAT_COUNT

Currently the documentation of GIT_PERF_REPEAT_COUNT says the default is
five while "perf-lib.sh" uses a value of three as a default.

Update the documentation so that it is consistent with the code.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/README

@@ -56,7 +56,7 @@ You can set the following variables (also in your config.mak):
 
     GIT_PERF_REPEAT_COUNT
 	Number of times a test should be repeated for best-of-N
-	measurements.  Defaults to 5.
+	measurements.  Defaults to 3.
 
     GIT_PERF_MAKE_OPTS
 	Options to use when automatically building a git tree for