Added list of os page. Fixed mistake in website editing docs about xml

syntax.

docs/editing-howto.xml

@@ -63,7 +63,7 @@
       file. We add the two lines:
     </p>
     <code>
-      &lt;xml version='1.0'&gt;<br />
+      &lt;&#143;xml version='1.0'&#143;&gt;<br />
       &lt;!DOCTYPE cscpage SYSTEM &quot;../csc.dtd&quot;&gt;
     </code>
     <p>

office/Makefile

@@ -1,5 +1,5 @@
 SUBDIRS = books
-INPUTS = index.xml staff.xml books.xml
+INPUTS = index.xml staff.xml books.xml operating-systems.xml
 
 include ../default.mk
 

office/directory.xml

@@ -5,4 +5,5 @@
 <directory title="Office">
   <diritem title="Staff" href="staff.html" />
   <diritem title="Books" href="books.html" />
+  <diritem title="Operating Systems" href="operating-systems.html" />
 </directory>