Typos (#4851)

# Fixed typos (issues #4818 & #4668 & more typos)
- At some places, it said `model = ChatOpenAI(model='gpt-3.5-turbo')`
but should be `model = ChatOpenAI(model_name='gpt-3.5-turbo')`
- Fixes some other typos

Fixes #4818, #4668

## Who can review?
Community members can review the PR once tests pass. Tag
maintainers/contributors who might be interested:
        Models
        - @hwchase17
        - @agola11
        Agents / Tools / Toolkits
        - @vowelparrot

docs/modules/agents/agents/custom_agent.ipynb

@@ -1,6 +1,7 @@
 {
  "cells": [
   {
+   "attachments": {},
    "cell_type": "markdown",
    "id": "ba5f8741",
    "metadata": {},
@@ -9,7 +10,7 @@
     "\n",
     "This notebook goes through how to create your own custom agent.\n",
     "\n",
-    "An agent consists of three parts:\n",
+    "An agent consists of two parts:\n",
     "    \n",
     "    - Tools: The tools the agent has available to use.\n",
     "    - The agent class itself: this decides which action to take.\n",