(documentation change) Specify full path to /usr/bin/xattr for macOS, f…

…ixes #2583

On some Macs, Homebrew installs its own `xattr` which doesn't have the `-r` option. See #2583.

This PR just adds the full path to the system `xattr` in `README.md`, so the `-r` in the instructions doesn't cause a problem.

It's not clear to me whether or not the `-r` is required; this PR does not address that question, it just routes around it.

I have tested the instruction on my Mac which has the Homebrew `xattr`, and with the added path, it works.

I would assume this change is low risk; it's just to documentation, and the change points to a system command that should always be there. However, it's possible I'm missing corner cases that further testing and validation on more machines would uncover.

README.md

@@ -192,7 +192,7 @@ brew install th-ch/youtube-music/youtube-music
 If you install the app manually and get an error "is damaged and can’t be opened." when launching the app, run the following in the Terminal:
 
 ```bash
-xattr -cr /Applications/YouTube\ Music.app
+/usr/bin/xattr -cr /Applications/YouTube\ Music.app
 ```
 
 ### Windows