Use correct name and URL for MinGW-w64, error noticed by Gurjeet Singh.

doc/src/sgml/install-windows.sgml

@@ -48,7 +48,7 @@
   <xref linkend="installation"> and the specific notes in
   <xref linkend="installation-notes-mingw"> and <xref linkend="installation-notes-cygwin">.
   To produce native 64 bit binaries in these environments, use the tools from
-  <productname>Mingw64</productname>. These tools can also be used to
+  <productname>MinGW-w64</productname>. These tools can also be used to
   cross-compile for 32 bit and 64 bit <productname>Windows</productname> 
   targets on other hosts, such as <productname>Linux</productname> and 
   <productname>Darwin</productname>.

doc/src/sgml/installation.sgml

@@ -2600,7 +2600,7 @@ cc-1020 cc: ERROR File = pqcomm.c, Line = 427
 
    <para>
      To build 64 bit binaries using MinGW, install the 64 bit tool set
-     from <ulink url="http://www.mingw64.org/"></ulink>, put its bin
+     from <ulink url="http://mingw-w64.sourceforge.net/"></ulink>, put its bin
      directory in the <envar>PATH</envar>, and run 
      <command>configure</command> with the  
      <command>--host=x86_64-w64-mingw</command> option.