chore(demo): bug in google_fonts causing compile error

Bug in google_fonts causing compilation error of example & widgetbook
`Error: Type 'FontFeature' not found.`

Author: benken

example/pubspec.yaml

@@ -12,7 +12,7 @@ dependencies:
   flutter:
     sdk: flutter
   go_router: ^11.1.2
-  google_fonts: ^6.1.0
+  google_fonts: 6.1.0
   path_drawing: ^1.0.1
   shared_preferences: ^2.2.2
   zeta_flutter: