drive-by fix to error string generation

We're using a formatting string, but not the `f` variant of `Sprint`.

docker/config/config.go

@@ -119,7 +119,7 @@ func Load(fileName string) (*Config, error) {
 		if fileName != DefaultDockerJSON {
 			errStr := "Invalid auth for Docker registry: %s\nBase64-encoded string is wrong: %s (%s)\n"
 			return nil, errors.New(
-				fmt.Sprint(
+				fmt.Sprintf(
 					errStr,
 					registry,
 					a.B64Auth,