add ref

Author: mongoKart

source/tools.txt

@@ -260,6 +260,8 @@ daemon in the global application group:
    and in the `Multiple Python Sub Interpreters <https://modwsgi.readthedocs.io/en/master/user-guides/application-issues.html#multiple-python-sub-interpreters>`__
    section of the mod_wsgi documentation.
 
+.. _pymongo-tools-type-checkers:
+
 Type Checkers
 -------------