change redirect URI due to Spotify Developer dashboard changes.

The Spotify Developer Dashboard no longer allows the use of http unless
it's to loopback address. It does not work even with "localhost".

README.md

@@ -167,7 +167,7 @@ In order for searching to work, you need to set up your own Spotify Developer ap
 4. Fill in:
    - App name (e.g., "My Spotizerr App")
    - App description
-   - Redirect URI: `http://localhost:7171/callback` (or your custom domain if exposed)
+   - Redirect URI: `http://127.0.0.1:7171/callback` (or your custom domain if exposed)
    - Check the Developer Terms agreement box
 5. Click "Create"
 6. On your app page, note your "Client ID"