Doc: fix overenthusiastic markup.

I get "WARNING: nested link may be undefined in output: <xref @linkend =
'pgbench'> nested inside parent element link" from this.

Also remove some trailing whitespace.

doc/src/sgml/release-11.sgml

@@ -2119,7 +2119,7 @@ same commits as above
     </sect4>
 
     <sect4>
-     <title><link linkend="pgbench"><xref linkend="pgbench"/></link></title>
+     <title><link linkend="pgbench"><application>pgbench</application></link></title>
 
      <itemizedlist>