C++ README: fix link to tutorial

Supersedes #20996, which was approved in 2019/11 but still doesn't have a signed CLA.

src/cpp/README.md

@@ -157,12 +157,12 @@ You can find out how to build and run our simplest gRPC C++ example in our
 For more detailed documentation on using gRPC in C++ , see our main
 documentation site at [grpc.io](https://grpc.io), specifically:
 
-* [Overview](https://grpc.io/docs/): An introduction to gRPC with a simple
+* [Overview](https://grpc.io/docs): An introduction to gRPC with a simple
   Hello World example in all our supported languages, including C++.
-* [gRPC Basics - C++](https://grpc.io/docs/tutorials/basic/c.html):
+* [gRPC Basics - C++](https://grpc.io/docs/tutorials/basic/cpp):
   A tutorial that steps you through creating a simple gRPC C++ example
   application.
-* [Asynchronous Basics - C++](https://grpc.io/docs/tutorials/async/helloasync-cpp.html):
+* [Asynchronous Basics - C++](https://grpc.io/docs/tutorials/async/helloasync-cpp):
   A tutorial that shows you how to use gRPC C++'s asynchronous/non-blocking
   APIs.