Correct usage of formatted output in a unit test.

pkg/rest/peminfo_handler_test.go

@@ -650,7 +650,7 @@ func TestGetPem(t *testing.T) {
 			"got %s want %s", problem.Title, errPemNotFound.Title)
 	}
 	if !strings.Contains(problem.Detail, "version=0815") {
-		t.Error("GET /v1/pems/ns1/cafe?version problem detail got %s "+
+		t.Errorf("GET /v1/pems/ns1/cafe?version problem detail got %s "+
 			"want contains version=0815", problem.Detail)
 	}
 	if problem.Status != http.StatusNotFound {