make Map.create take Brr.El

example/dune

@@ -1,7 +1,7 @@
 (executable
  (name main)
  (modules main)
- (libraries leaflet)
+ (libraries brr leaflet)
  (modes js)
  (preprocess
   (pps js_of_ocaml-ppx)))

example/main.ml

@@ -1,7 +1,11 @@
 (* create map *)
 let map =
-  let container_id = "map" in
-  Leaflet.Map.create container_id
+  let container =
+    match Brr.Document.find_el_by_id Brr.G.document (Jstr.of_string "map") with
+    | None -> failwith "map container not found"
+    | Some el -> el
+  in
+  Leaflet.Map.create container
 
 (* setup map *)
 let () =