DOC: added missing literal tags

Author: Liudmila Mantrova

doc/src/sgml/recovery-config.sgml

@@ -14,7 +14,8 @@
     <filename>recovery.conf</><indexterm><primary>recovery.conf</></>
     file. They apply only for the duration of the
     recovery.  They must be reset for any subsequent recovery you wish to
-    perform.  The file content is reloaded whenever the main server process receives a SIGHUP signal.
+    perform.  The file content is reloaded whenever the main server
+    process receives a <literal>SIGHUP</literal> signal.
    </para>
 
    <para>