DOC: fix wrong copy-paste in the rspdel example

As reported by Cristian Ditoiu, the rspdel example is based on the reqrep
action, which is quite confusing!
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 9fed5c8..4561741 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -5011,7 +5011,7 @@
 
   Example :
      # remove the Server header from responses
-     reqidel ^Server:.*
+     rspidel ^Server:.*
 
   See also: "rspadd", "rsprep", "reqdel", section 6 about HTTP header
             manipulation, and section 7 about ACLs.