fix Stacktrace example
diff --git a/example_test.go b/example_test.go
index c763caa..427e2a9 100644
--- a/example_test.go
+++ b/example_test.go
@@ -70,7 +70,7 @@
 	// Output: github.com/pkg/errors/example_test.go:67: whoops: foo
 }
 
-func ExampleError_Stacktrace() {
+func Example_stacktrace() {
 	type Stacktrace interface {
 		Stacktrace() []errors.Frame
 	}