runtime: fix backward error message

This was added in CL 339990.

Change-Id: I4b0f97bf1a3926e37a42f77e149dcab3b7b75a63
Reviewed-on: https://go-review.googlesource.com/c/go/+/359255
Trust: Michael Pratt <[email protected]>
Run-TryBot: Michael Pratt <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: prattmic

src/runtime/crash_cgo_test.go

@@ -534,7 +534,7 @@ func TestCgoTracebackSigpanic(t *testing.T) {
 	// No runtime errors like "runtime: unexpected return pc".
 	nowant := "runtime: "
 	if strings.Contains(got, nowant) {
-		t.Errorf("unexpectedly saw %q in output", want)
+		t.Errorf("unexpectedly saw %q in output", nowant)
 	}
 }