update cxx url path to cpp (#162)

Author: rachel-mack

source/index.txt

@@ -40,7 +40,7 @@ MongoDB Documentation
 
          .. card::
             :headline: C++
-            :url: https://www.mongodb.com/docs/drivers/cxx/
+            :url: https://www.mongodb.com/docs/drivers/cpp/
             :icon: /images/icons/cpp.svg
             :icon-alt: C plus plus Driver icon