fix(flags): add missing provider line to openfeature python code example (#14248)

Openfeature needs a provider to evaluate flags.

Author: aliu39

docs/platforms/python/integrations/openfeature/index.mdx

@@ -47,6 +47,7 @@ The integration is tested by evaluating a feature flag using your OpenFeature SD
 from openfeature import api
 import sentry_sdk
 
+api.set_provider(MyProviderOfChoice(...))
 client = api.get_client()
 client.get_boolean_value("hello", default_value=False)