Add <literal> markup, for consistency.

This file isn't entirely consistent about whether "on" and "off"
should be marked up with <literal>, but it doesn't make much sense
to be inconsistent within a single sentence.

Etsuro Fujita

doc/src/sgml/config.sgml

@@ -6752,7 +6752,7 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
         determines whether OIDs will be included in tables created by
         <command>SELECT INTO</command>. The parameter is <literal>off</>
         by default; in <productname>PostgreSQL</> 8.0 and earlier, it
-        was on by default.
+        was <literal>on</> by default.
        </para>
 
        <para>