Convert readthedocs link for their .org -> .io migration for hosted projects

As per their email ‘Changes to project subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

Author: Adam Chainz

docs/proposals/functions.rst

@@ -11,7 +11,7 @@ Abstract
 ========
 
 This document proposes modifying the
-`JMESPath grammar <http://jmespath.readthedocs.org/en/latest/specification.html#grammar>`__
+`JMESPath grammar <https://jmespath.readthedocs.io/en/latest/specification.html#grammar>`__
 to support function expressions.
 
 Motivation

docs/proposals/nested-expressions.rst

@@ -10,7 +10,7 @@ Nested Expressions
 Abstract
 ========
 
-This document proposes modifying the `JMESPath grammar <http://jmespath.readthedocs.org/en/latest/specification.html#grammar>`_
+This document proposes modifying the `JMESPath grammar <https://jmespath.readthedocs.io/en/latest/specification.html#grammar>`_
 to support arbitrarily nested expressions within ``multi-select-list`` and
 ``multi-select-hash`` expressions.