doc: recommend benchstat for performance commits

The benchstat tool computes statistics about benchmarks, including
whether any differences are statistically significant. Recommend its use
in commit messages of performance-related changes rather than the
simpler benchcmp tool.

Change-Id: I4b35c2d892b48e60c3064489b035774792c19c30
Reviewed-on: https://go-review.googlesource.com/132515Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -696,7 +696,7 @@ Don't use HTML, Markdown, or any other markup language.
 <p>
 Add any relevant information, such as benchmark data if the change
 affects performance.
-The <a href="https://godoc.org/golang.org/x/tools/cmd/benchcmp">benchcmp</a>
+The <a href="https://godoc.org/golang.org/x/perf/cmd/benchstat">benchstat</a>
 tool is conventionally used to format
 benchmark data for change descriptions.
 </p>