Add fonts ttf files in example

https://stackoverflow.com/questions/55075834/fontfamily-property-not-working-properly-in-flutter and https://www.flutterbeads.com/change-font-family-flutter/

example/pubspec.yaml

@@ -56,7 +56,16 @@ flutter:
   assets:
     - assets/
 
-
+  fonts:
+    - family: monospace
+      fonts:
+        - asset: assets/fonts/MonoSpace.ttf
+    - family: serif
+      fonts:
+        - asset: assets/fonts/Serif.ttf
+    - family: sans-serif
+      fonts:
+        - asset: assets/fonts/SansSerif.ttf
   # An image asset can refer to one or more resolution-specific "variants", see
   # https://flutter.dev/assets-and-images/#resolution-aware.