Escape slashes in class name

Author: jmikola

source/drivers/php.txt

@@ -107,7 +107,7 @@ Compatibility
 Due to potential problems representing 64-bit integers on 32-bit platforms,
 users are advised to use 64-bit environments. When using a 32-bit platform, be
 aware that any 64-bit integer read from the database will be returned as a
-`MongoDB\BSON\Int64 <https://www.php.net/manual/en/class.mongodb-bson-int64.php>`_
+`MongoDB\\BSON\\Int64 <https://www.php.net/manual/en/class.mongodb-bson-int64.php>`_
 instance instead of a PHP integer type.
 
 MongoDB Compatibility