endpoints: fix %q verb use with wrong type

Caught early by the improved vet check gated behind the 1.26 language
version combined with a tiplang builder that tests with 1.26 language
version.

Change-Id: Ie41542014acbbf24464111bd8dabb040ab0a0dea
Cq-Include-Trybots: luci.golang.try:x_oauth2-gotip-linux-amd64-tiplang
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/725240
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>

Author: dmitshur

endpoints/endpoints_test.go

@@ -36,7 +36,7 @@ func TestAWSCognitoEndpoint(t *testing.T) {
 		t.Run(tt.in, func(t *testing.T) {
 			endpoint := AWSCognito(tt.in)
 			if endpoint != tt.out {
-				t.Errorf("got %q, want %q", endpoint, tt.out)
+				t.Errorf("got %#v, want %#v", endpoint, tt.out)
 			}
 		})
 	}