testing: fix testing flags link

Fixed a broken link to a section in the documentation for the
test flags for the go command.

Change-Id: Ic4bdd4965aac7856dd13a2adda9d774b9bae4113
GitHub-Last-Rev: 15bda340674c268a92f43a2820eca50482940de1
GitHub-Pull-Request: golang/go#24613
Reviewed-on: https://go-review.googlesource.com/103835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/testing.go

@@ -34,7 +34,7 @@
 // its -bench flag is provided. Benchmarks are run sequentially.
 //
 // For a description of the testing flags, see
-// https://golang.org/cmd/go/#hdr-Description_of_testing_flags.
+// https://golang.org/cmd/go/#hdr-Testing_flags
 //
 // A sample benchmark function looks like this:
 //     func BenchmarkHello(b *testing.B) {