Final instead of var and improve progress

Author: hamishdgx

Progress.md

@@ -62,7 +62,7 @@ Dart. This is an on going project and functions are being added once needed. If
 
 - [ ] combine
 - [x] [explode](https://github.com/dartclub/turf_dart/blob/main/lib/src/explode.dart)
-- [x] flatten
+- [x] [flatten](https://github.com/dartclub/turf_dart/blob/main/lib/src/flatten.dart)
 - [x] [lineToPolygon](https://github.com/dartclub/turf_dart/blob/main/lib/src/line_to_polygon.dart)
 - [ ] polygonize
 - [x] [polygonToLine](https://github.com/dartclub/turf_dart/blob/main/lib/src/polygon_to_line.dart)

lib/src/flatten.dart

@@ -17,12 +17,12 @@ import 'package:turf/src/meta/flatten.dart';
 ///
 /// Example:
 /// ```dart
-/// var multiLineString = MultiLineString(coordinates: [
+/// final multiLineString = MultiLineString(coordinates: [
 ///   [Position(0, 0), Position(1, 1)],
 ///   [Position(2, 2), Position(3, 3)]
 /// ]);
 ///
-/// var flattened = flatten(multiLineString);
+/// final flattened = flatten(multiLineString);
 /// // Returns FeatureCollection with 2 LineString features
 /// ```
 ///