packaging: replace a documentation reference to `python3` on Windows

Sadly, there's not `python3.exe` on Windows.

Differential Revision: https://phab.mercurial-scm.org/D9937

contrib/packaging/inno/readme.rst

@@ -34,7 +34,7 b' From the prompt, change to the Mercurial'
 Next, invoke ``packaging.py`` to produce an Inno installer. You will
 need to supply the path to the Python interpreter to use.::
 
-   $ python3.exe contrib\packaging\packaging.py \
+   $ py -3 contrib\packaging\packaging.py \
        inno --python c:\python27\python.exe
 
 .. note::

contrib/packaging/wix/readme.rst

@@ -40,7 +40,7 b' From the prompt, change to the Mercurial'
 Next, invoke ``packaging.py`` to produce an MSI installer. You will need
 to supply the path to the Python interpreter to use.::
 
-   $ python3 contrib\packaging\packaging.py \
+   $ py -3 contrib\packaging\packaging.py \
       wix --python c:\python27\python.exe
 
 .. note::