Rename confusing var

.env.example

@@ -1,3 +1,3 @@
 # Currently needed by dev environment, will be replace by some kind of config file that's generated by nix
-DISCORD_CLIENT_ID=
+DISCORD_APP_ID=
 LB_USER=

src/main.rs

@@ -25,7 +25,7 @@ fn main() -> Result<(), Box<dyn Error>> {
     }
 
     let mut lb = listenbrainz::Client::new();
-    let mut drpc = Client::new(env::var("DISCORD_CLIENT_ID")?.parse()?);
+    let mut drpc = Client::new(env::var("DISCORD_APP_ID")?.parse()?);
     drpc.start();
 
     let mut cover_cache: HashMap<String, String> = HashMap::new();