Don't recommend upgrading to latest available Windows SDK

We only support up to version 7.0, so don't recommend
upgrading past it. The rest of the documentation around this
was already updated, but one spot was missed.

Author: mhagander

doc/src/sgml/install-windows.sgml

@@ -150,9 +150,10 @@ $ENV{PATH}=$ENV{PATH} . ';c:\some\where\bison\bin';
     <varlistentry>
      <term><productname>Microsoft Platform SDK</productname></term>
      <listitem><para>
-      It is recommended that you upgrade to the latest available version
-      of the <productname>Microsoft Platform SDK</productname>, available
-      for download from <ulink url="http://www.microsoft.com/downloads/"></>.
+      It is recommended that you upgrade to the latest supported version
+      of the <productname>Microsoft Platform SDK</productname> (currently
+      version 7.0), available for download from
+      <ulink url="http://www.microsoft.com/downloads/"></>.
      </para>
      <para>
       You must always include the