Add missing result marker in Inspect moduledoc (#15577)

Author: AlexGx

lib/elixir/lib/inspect.ex

@@ -92,7 +92,7 @@ defprotocol Inspect do
       end
 
       inspect(%Point{x: 1})
-      %Point{x: 1, y: 0}
+      #=> %Point{x: 1, y: 0}
 
   ## Custom implementation