Fixing grammar issues (#5537)

Fixing grammar issues

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python/packages/autogen-core/docs/src/user-guide/agentchat-user-guide/custom-agents.ipynb

@@ -708,7 +708,7 @@
     "So far, we have seen how to create custom agents, add custom model clients to agents, and make custom agents declarative. There are a few ways in which this basic sample can be extended:\n",
     "\n",
     "- Extend the Gemini model client to handle function calling similar to the {py:class}`~autogen_agentchat.agents.AssistantAgent` class. https://ai.google.dev/gemini-api/docs/function-calling  \n",
-    "- Implement a package wit a custom agent and experiment with using it's declarative format in a tool like [AutoGen Studio](https://microsoft.github.io/autogen/stable/user-guide/autogenstudio-user-guide/index.html)."
+    "- Implement a package with a custom agent and experiment with using its declarative format in a tool like [AutoGen Studio](https://microsoft.github.io/autogen/stable/user-guide/autogenstudio-user-guide/index.html)."
    ]
   }
  ],