Update README.txt

Cpp extension link update

Author: sekyondaMeta

advanced_source/README.txt

@@ -9,6 +9,6 @@ Advanced Tutorials
 	Creating Extensions Using numpy and scipy
 	https://pytorch.org/tutorials/advanced/numpy_extensions_tutorial.html
 
-3. c_extension.rst
+3. cpp_extension.rst
 	Custom C Extensions for PyTorch
-	https://pytorch.org/tutorials/advanced/c_extension.html
+	https://docs.pytorch.org/tutorials/advanced/cpp_extension.html