Fix OpenRC command for adding grub-btrfsd to startup

Fixes #427 
Replace `rc-config add` with `rc-update add` in the README.

Author: Antynea

README.md

@@ -240,7 +240,7 @@ sudo rc-service grub-btrfsd start
 
 To activate it during system startup, run:
 ```bash
-sudo rc-config add grub-btrfsd default 
+sudo rc-update add grub-btrfsd default 
 ```
 
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