Clarify note on intmath's PyPI package

Author: skirpichev

peps/pep-0791.rst

@@ -150,8 +150,8 @@ Module functions will accept integers and objects that implement the
 object to an integer number.  Suitable functions must be computed exactly,
 given sufficient time and memory.
 
-The :pypi:`intmath` package will provide new submodule content for older Python
-versions.
+The :pypi:`intmath` package, available on PyPI, will provide new submodule
+content for older Python versions.
 
 
 Possible Extensions