Fix docs for Process.monitor/2 (#14009)

Previously, the example contained 2 issues.

1. A syntax error on `receive` as there were no parens.
2. The example was incomplete, and would hang on the 2nd receive

This commit updates the example to reflect the original intent, by
sending 2 messages and getting a `:noproc` on the 2nd `receive`.

Author: Gazler

lib/elixir/lib/process.ex

@@ -569,10 +569,15 @@ defmodule Process do
 
       send(pid, {:ping, ref_and_alias})
 
-      receive do: msg -> msg
+      receive do: (msg -> msg)
       #=> :pong
 
-      receive do: msg -> msg
+      ref_and_alias = Process.monitor(pid, alias: :reply_demonitor)
+      #=> #Reference<0.906660723.3006791681.40191>
+
+      send(pid, {:ping, ref_and_alias})
+
+      receive do: (msg -> msg)
       #=> {:DOWN, #Reference<0.906660723.3006791681.40191>, :process, #PID<0.118.0>, :noproc}
 
   """