docs: Update add-function.md to fix example (#6673)

When performing the changes described in add-function.md for adding a
host-price-of function to xapi, a type error would arise from the
message-forwarding.ml file - this is fixed by explicitly giving the
remote_fn named argument.

Author: robhoes

doc/content/xapi/guides/howtos/add-function.md

@@ -172,8 +172,8 @@ the Host module:
     let price_of ~__context ~host ~item =
         info "Host.price_of for item %s" item;
         let local_fn = Local.Host.price_of ~host ~item in
-        do_op_on ~local_fn ~__context ~host
-          (fun session_id rpc -> Client.Host.price_of ~rpc ~session_id ~host ~item)
+        let remote_fn = Client.Host.price_of ~host ~item in
+        do_op_on ~local_fn ~__context ~host ~remote_fn
 
 After the ~__context parameter, the parameters of this new function should
 match the parameters we specified for the message. In this case, that is the