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>