Slightly improve fixed rules description for Biba

svn path=/head/; revision=21450

en_US.ISO8859-1/books/handbook/mac/chapter.sgml

@@ -1428,9 +1428,9 @@ test: biba/high</screen>
       <listitem>
 	<para>Fixed rules: no write up, no read down (opposite of
 	  <acronym>MLS</acronym>).  A subject can have write access
-	  to objects on its own level but not above.  Similarly, a
+	  to objects on its own level or below, but not above.  Similarly, a
 	  subject can have read access to objects on its own level
-	  above but not below;</para>
+	  or above, but not below;</para>
       </listitem>
 
       <listitem>