Update path to Blink perf tests documentation

Change-Id: I3c085d43bc1fa2b3ebe3d90c50b3d1e269de58a0
Reviewed-on: https://chromium-review.googlesource.com/1054289Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Manuel Rego Casasnovas <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#557825}

third_party/blink/perf_tests/README.md

-Please refer to [blink_perf guide](../../../docs/speed/benchmark_harnesses/blink_perf.md).
+Please refer to [blink_perf guide](../../../docs/speed/benchmark/harnesses/blink_perf.md).