OpenRC deprecated instuctions in README.md

```
grub-btrfsd OpenRC instructions

To start the daemon run:

sudo rc-service grub-btrfsd start 

To activate it during system startup, run:

sudo rc-config add grub-btrfsd default 
```
the final line `sudo rc-config add grub-btrfsd default`

should be changed to:
```
sudo rc-update add grub-btrfsd default
```