fix small type violation

error:

    warning: incompatible types given to Kernel.hd/1:

        hd(__CALLER__.context_modules)

    given types:

        list(atom())

    but expected one of:

        non_empty_list(term(), term())

    typing violation found at:
    │
  3 │     module = hd(__CALLER__.context_modules)
    │              ~
    │
    └─ lib/walex/event/dsl.ex:3:14: WalEx.Event.Dsl.process_events_async/2

lib/walex/event/dsl.ex

@@ -1,6 +1,7 @@
 defmodule WalEx.Event.Dsl do
   defmacro process_events_async(events, functions) do
-    module = hd(__CALLER__.context_modules)
+    [_ | _] = __CALLER__.context_modules
+    module = List.first(__CALLER__.context_modules)
 
     quote do
       Enum.each(unquote(events), fn event ->