docs: update api-doc link to version 3.6 (#2640)

The link to the API docs in the README.md for 3.6 was incorrectly pointing
to the 3.1 api docs.

Author: franciscofabian

README.md

@@ -15,7 +15,7 @@ The official [MongoDB](https://www.mongodb.com/) driver for Node.js. Provides a
 | what          | where                                          |
 |---------------|------------------------------------------------|
 | documentation | http://mongodb.github.io/node-mongodb-native  |
-| api-doc        | http://mongodb.github.io/node-mongodb-native/3.1/api  |
+| api-doc        | http://mongodb.github.io/node-mongodb-native/3.6/api  |
 | source        | https://github.com/mongodb/node-mongodb-native |
 | mongodb       | http://www.mongodb.org                        |