docs: fix nebula description at https://python.langchain.com/docs/integrations/chat/ All chat models section (#31315)

…egrations/chat/ All chat models section

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Author: MichaelLi65535

docs/docs/integrations/chat/symblai_nebula.ipynb

@@ -21,7 +21,6 @@
    "source": [
     "# Nebula (Symbl.ai)\n",
     "\n",
-    "## Overview\n",
     "This notebook covers how to get started with [Nebula](https://docs.symbl.ai/docs/nebula-llm) - Symbl.ai's chat model.\n",
     "\n",
     "### Integration details\n",