docs: agents & callbacks fixes (#10066)

Various improvements to the Agents & Callbacks sections of the
documentation including formatting, spelling, and grammar fixes to
improve readability.

docs/extras/modules/agents/how_to/custom-functions-with-openai-functions-agent.ipynb

@@ -17,9 +17,11 @@
     "id": "LFKylC3CPtTl"
    },
    "source": [
-    "Install libraries which are required to run this example notebook\n",
+    "Install libraries which are required to run this example notebook:\n",
     "\n",
-    "`pip install -q openai langchain yfinance`"
+    "```bash\n",
+    "pip install -q openai langchain yfinance\n",
+    "```\n"
    ]
   },
   {