pkg: fix incorrect prints found by govet

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/5266041
diff --git a/src/pkg/template/exec_test.go b/src/pkg/template/exec_test.go
index 46b89fd..d583033 100644
--- a/src/pkg/template/exec_test.go
+++ b/src/pkg/template/exec_test.go
@@ -532,7 +532,7 @@
 			t.Fatalf("delim %q exec err %s", left, err)
 		}
 		if b.String() != hello+trueLeft {
-			t.Error("expected %q got %q", hello+trueLeft, b.String())
+			t.Errorf("expected %q got %q", hello+trueLeft, b.String())
 		}
 	}
 }