Don't mention the reference part of the mypy docs (#1800)

Author: srittau

docs/index.rst

@@ -21,9 +21,7 @@ Reference
 .. seealso::
 
    The documentation at https://mypy.readthedocs.io/ is relatively accessible
-   and complete. Particularly refer to the "Type System Reference" section of
-   the docs -- since the Python typing system is standardised via PEPs, this
-   information should apply to most Python type checkers.
+   and complete.
 
 Specification
 =============