doc: remove mention of Windows junction points by pg_upgrade

pg_upgrade never used Windows junction points but instead always used
Windows hard links.

Reported-by: Adrian Klaver

Discussion: https://postgr.es/m/6a638c60-90bb-4921-8ee4-5fdad68f8b09@aklaver.com

Backpatch-through: 9.3, where the mention first appeared

doc/src/sgml/ref/pgupgrade.sgml

@@ -123,7 +123,7 @@
       <term><option>-k</option></term>
       <term><option>--link</option></term>
       <listitem><para>use hard links instead of copying files to the new
-      cluster (use junction points on Windows)</para></listitem>
+      cluster</para></listitem>
      </varlistentry>
 
      <varlistentry>