Fix duplicate label and a typo.

Doc/using/unix.rst

 .. highlightlang:: none
 
-.. _using-on-windows:
+.. _using-on-unix:
 
 ********************************
  Using Python on Unix platforms
@@ -79,9 +79,10 @@ extensively documented in the :file:`README` file in the root of the Python
 source tree.
 
 .. warning::
-   ``make install`` can overwrite or masquerade the :file:`python` binary;
-   ``make altinstall`` is recommended instead of ``make install`` since it
-   only install :file:`{exec_prefix}/bin/python{version}`.
+
+   ``make install`` can overwrite or masquerade the :file:`python` binary.
+   ``make altinstall`` is therefore recommended instead of ``make install``
+   since it only installs :file:`{exec_prefix}/bin/python{version}`.
 
 
 Python-related paths and files