updated index.md

Author: aryamshabbir

docs/index.md

@@ -26,7 +26,7 @@ This documentation is organized into specialized sections. Click on any page bel
 
 - [ Home](home.md)
 - [ Installation](installation.md)
-- [Theory of Operation](theory_of_operation.md)
+- [Theory of Operation](https://github.com/meds-ee-uet/CAN-Bus/blob/main/docs/theory_of_operation.md)
 - [ API Reference](API_Reference.md)
 - [ Contributing](contributing.md)