`google_maps`: Tidyup code formatting (#1723)

google_maps/lib/main.dart

@@ -50,12 +50,12 @@ class _MyAppState extends State<MyApp> {
             zoom: 11.0,
           ),
           markers: {
-              // ignore: prefer_const_constructors
+            const Marker(
-              Marker(
+              markerId: MarkerId('Sydney'),
-                markerId: const MarkerId('Sydney'),
+              position: LatLng(-33.86, 151.20),
-                position: const LatLng(-33.86, 151.20),
             )
-            }),
+          },
+        ),
       ),
     );
   }