ssh: fix a formatting bug in a test

Change-Id: I34cc1a5a8c59c5e4c0313d7a41c3f31cdbad9e98
Reviewed-on: https://go-review.googlesource.com/47933
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/client_auth_test.go

@@ -622,7 +622,7 @@ func TestClientAuthErrorList(t *testing.T) {
 				t.Fatalf("errors: got %v, want %v", e, publicKeyErr)
 			}
 		default:
-			t.Fatal("errors: got %v, expected 2 errors", authErrs.Errors)
+			t.Fatalf("errors: got %v, expected 2 errors", authErrs.Errors)
 		}
 	}
 }