Driver release 2.6.3 was skipped, update the docs to point to 2.6.4 instead
Driver release 2.6.3 was skipped, update the docs to point to 2.6.4 instead

mongodb/mongo-ruby-driver,jonhyman/mongo-ruby-driver,mongodb/mongo-ruby-driver,mongodb/mongo-ruby-driver,jonhyman/mongo-ruby-driver