chat.tmux.conf: use config_files format for reload

The recent addition of "#{source_files}" allows us to avoid hardcoding
the file name and instead ask tmux itself for the very file it used to
create the session in the first place, i.e. "-f ./chat.tmux.conf".

scripts/chat.tmux.conf

@@ -49,7 +49,7 @@ bind-key -n -N 'toggle fullscreen'	-- F11	set status
 # this configuration is idempotent, i.e. reloading it only changes settings
 # and never duplicates already existing windows
 bind-key -N 'reload configuration'	-- R	{
-	source-file	-- ./chat.tmux.conf
+	source-file -F	-- '#{source_files}'
 	display-message	-- 'configuration reloaded'
 }