testing: use -bench flag and fix flags doc link

R=golang-dev, r, kr
CC=golang-dev
https://golang.org/cl/10263043

src/pkg/testing/testing.go

@@ -23,10 +23,10 @@
 // Functions of the form
 //     func BenchmarkXxx(*testing.B)
 // are considered benchmarks, and are executed by the "go test" command when
-// the -test.bench flag is provided. Benchmarks are run sequentially.
+// its -bench flag is provided. Benchmarks are run sequentially.
 //
 // For a description of the testing flags, see
-// http://golang.org/cmd/go/#Description_of_testing_flags.
+// http://golang.org/cmd/go/#hdr-Description_of_testing_flags.
 //
 // A sample benchmark function looks like this:
 //     func BenchmarkHello(b *testing.B) {