Fix filtering documentation

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.

Author: scohen

lib/logger/lib/logger.ex

@@ -436,8 +436,8 @@ 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}} ->
+              if ensure_binary(msg) =~ "password" do
                 :stop
               else
                 :ignore
@@ -447,6 +447,14 @@ defmodule Logger do
               :ignore
           end
         end
+
+        defp ensure_binary(charlist) when is_list(charlist) do
+          List.to_string(charlist)
+        end
+
+        defp ensure_binary(string) when is_binary(string) do
+          string
+        end
       end
 
   The filter may return the possibly modified event (to change the