fix typo in config example variable (#209)

Author: hyperpuncher

examples/app.toml

@@ -1,7 +1,7 @@
 theme = "default"
 client_id = "65b708073fc0480ea92a077233ca87bd"
 client_port = 8080
-playack_format = "{track} • {artists}\n{album}\n{metadata}"
+playback_format = "{track} • {artists}\n{album}\n{metadata}"
 notify_format = { summary = "{track} • {artists}", body = "{album}" }
 # the default `copy_command` is based on the OS
 copy_command = { command = "pbcopy", args = [] } # macos