Update the link to search4e.ipynb (aimacode#1083)

The "search-4e.ipynb" in line 21 is a typo for search4e.ipynb. File search-4e.ipynb does not exist. It will show 404 error.

Author: qinhuah

index.ipynb

@@ -18,7 +18,7 @@
     "\n",
     "3. [**Search**](./search.ipynb)\n",
     "\n",
-    "4. [**Search - 4th edition**](./search-4e.ipynb)\n",
+    "4. [**Search - 4th edition**](./search4e.ipynb)\n",
     "\n",
     "4. [**Games**](./games.ipynb)\n",
     "\n",