Doc change to make atom keys explicit for dynamic attributes (#3655)

Author: andrewtimberlake

lib/phoenix_component.ex

@@ -707,6 +707,7 @@ defmodule Phoenix.Component do
 
   In this case, the expression inside `{...}` must be either a keyword list or
   a map containing the key-value pairs representing the dynamic attributes.
+  If using a map, ensure your keys are atoms.
 
   ### Interpolating blocks