fix custom code doc link

Author: marktayl1

README.md

@@ -179,7 +179,7 @@ az containerapp create \
 
 ### How to customize
 
-This solution is designed to be easily customizable. You can modify the front end site, or even build your own front end and attach to the backend API. You can further customize the backend by adding your own agents with their own specific capabilities. Deeper technical information to aid in this customization can be found in this [document](.\documentation\CustomizeSolution.md).
+This solution is designed to be easily customizable. You can modify the front end site, or even build your own front end and attach to the backend API. You can further customize the backend by adding your own agents with their own specific capabilities. Deeper technical information to aid in this customization can be found in this [document](./documentation/CustomizeSolution.md).
 
 ### Additional resources