Fix provider documentation (#440)

Make example more realistic. This would break if you're running flask with multiple threads.

Author: sonthonaxrk

examples/providers/singleton_scoped.py

@@ -10,7 +10,7 @@ class Service:
 
 class Container(containers.DeclarativeContainer):
 
- service_provider = providers.Singleton(Service)
+ service_provider = providers.ThreadLocalSingleton(Service)
 
 
 def index_view():