Fixed property typo for p4counter.

Thanks to Bernd Gerlach-de Campos
PR: 30981


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@277029 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/OptionalTasks/perforce.html

@@ -567,7 +567,7 @@ Set the value of the counter based on the value of the "TSTAMP" proper
 Set the value of the "p4.last.clean.build" property to the current
 value of the "last-clean-build" counter:
 <PRE>
-&lt;p4counter name=&quot;last-clean-build&quot; property=&quot;${p4.last.clean.build}&quot;/&gt;
+&lt;p4counter name=&quot;last-clean-build&quot; property=&quot;p4.last.clean.build&quot;/&gt;
 </pre>
 <hr>