fix teams verify state handler in OAuthPrompt (#518) (#519)

Author: axelsrz

libraries/botbuilder-dialogs/botbuilder/dialogs/prompts/oauth_prompt.py

@@ -278,7 +278,7 @@ async def _recognize_token(self, context: TurnContext) -> PromptRecognizerResult
         if OAuthPrompt._is_token_response_event(context):
             token = context.activity.value
         elif OAuthPrompt._is_teams_verification_invoke(context):
-            code = context.activity.value.state
+            code = context.activity.value["state"]
             try:
                 token = await self.get_user_token(context, code)
                 if token is not None: