community:update telegram notebook (langchain-ai#18569)

**Description:** 
modified the user_name to username to conform with the expected inputs
to TelegramChatApiLoader

**Issue:**
Current code fails in langchain-community 0.0.24 
<loader = TelegramChatApiLoader(
    chat_entity="<CHAT_URL>",  # recommended to use Entity here
    api_hash="<API HASH >",
    api_id="<API_ID>",
    user_name="",  # needed only for caching the session.
)>

docs/docs/integrations/document_loaders/telegram.ipynb

@@ -80,7 +80,7 @@
     "    chat_entity=\"<CHAT_URL>\",  # recommended to use Entity here\n",
     "    api_hash=\"<API HASH >\",\n",
     "    api_id=\"<API_ID>\",\n",
-    "    user_name=\"\",  # needed only for caching the session.\n",
+    "    username=\"\",  # needed only for caching the session.\n",
     ")"
    ]
   },