[C++] Update C++ Docs URL slug from cxx to cpp (#954)

* cxx -> cpp

Author: rachel-mack

config/redirects

@@ -46,6 +46,8 @@ raw: docs/drivers/cpp-bson-helper-functions -> https://www.mongodb.com/docs/lang
 raw: docs/drivers/cpp-to-sql-to-mongo-shell -> https://www.mongodb.com/docs/languages/cpp/drivers/current/
 raw: docs/drivers/tutorial/download-and-compile-cpp-driver-legacy -> https://mongocxx.org/legacy-v1/
 raw: docs/drivers/tutorial/authenticate-with-cpp-driver -> https://www.mongodb.com/docs/languages/cpp/drivers/current/configuration/#configuring-authentication
+raw: docs/drivers/cxx -> docs/drivers/cpp
+raw: docs/drivers/cxx/index -> docs/drivers/cpp
 
 # csharp