Fix bad docs sidebar header (#7966)

Quick fix for:

<img width="283" alt="Screenshot 2023-07-19 at 2 49 44 PM"
src="https://github.com/hwchase17/langchain/assets/6952323/91e4868c-b75e-413d-9f8f-d34762abf164">

CC @baskaryan

docs/extras/modules/data_connection/document_transformers/post_retrieval/_category_.yml

@@ -0,0 +1 @@
+label: 'Post retrieval'

docs/extras/modules/data_connection/document_transformers/post_retrieval/long_context_reorder.ipynb

@@ -1,15 +1,16 @@
 {
  "cells": [
   {
+   "attachments": {},
    "cell_type": "markdown",
    "id": "fc0db1bc",
    "metadata": {},
    "source": [
     "# Lost in the middle: The problem with long contexts\n",
     "\n",
-    "No matter the architecture of your model, there is a sustancial performance degradation when you include 10+ retrieved documents.\n",
-    "In brief: When models must access relevant information  in the middle of long contexts, then tend to ignore the provided documents.\n",
-    "See: https://arxiv.org/abs//2307.03172\n",
+    "No matter the architecture of your model, there is a substantial performance degradation when you include 10+ retrieved documents.\n",
+    "In brief: When models must access relevant information in the middle of long contexts, then tend to ignore the provided documents.\n",
+    "See: https://arxiv.org/abs/2307.03172\n",
     "\n",
     "To avoid this issue you can re-order documents after retrieval to avoid performance degradation."
    ]