Fix filtering documentation (#14705)

The filtering documentation implied that the msg attribute of the
logger event map could be a binary, but according to the erlang types
(https://www.erlang.org/doc/apps/kernel/logger.html#t:log_event/0) it
can't be a binary.

This change updates the docs with an example that comports with the
actual typing.

lib/logger/lib/logger.ex

@@ -436,8 +436,9 @@ defmodule Logger do
       defmodule LogFilter do
         def filter(log_event, _opts) do
           case log_event do
-            %{msg: msg} when is_binary(msg) ->
-              if msg =~ "password" do
+            %{msg: {:string, msg}} ->
+              # msg may be a charlist or a binary
+              if to_string(msg) =~ "password" do
                 :stop
               else
                 :ignore