testing: fix minor bug with mutex profile error message

Change-Id: I92b7a44476cfd9e2f099477b9d7903bbaa2fd8da
Reviewed-on: https://go-review.googlesource.com/c/go/+/173720
Run-TryBot: Caleb Spare <cespare@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/testing/testing.go

@@ -1287,7 +1287,7 @@ func (m *M) writeProfiles() {
 			os.Exit(2)
 		}
 		if err = m.deps.WriteProfileTo("mutex", f, 0); err != nil {
-			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *blockProfile, err)
+			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *mutexProfile, err)
 			os.Exit(2)
 		}
 		f.Close()