Update firewall.adoc

Author: deniz-husaj

docs/modules/ROOT/pages/servlet/exploits/firewall.adoc

@@ -111,7 +111,7 @@ XML::
 ----
 <b:bean id="httpFirewall"
       class="org.springframework.security.web.firewall.StrictHttpFirewall"
-      p:allowedHttpMethods="GET,HEAD"/>
+      p:allowedHttpMethods="GET,POST"/>
 
 <http-firewall ref="httpFirewall"/>
 ----