The macOS shared directories documentations on mount_virtiofs is out of date

The [shared directories](https://docs.getutm.app/guest-support/macos/#shared-directories) documentation for macOS talks about:

> mount_virtiofs share [mount point]

inferring that `share` is an fs_tag that `mount_virtiofs` will recognize. However, trying that gave me the error:

```
mount_virtiofs: fs_tag share not found
```

The UTM source code now appears to [prefer using macOSGuestAutomountTag](https://github.com/utmapp/UTM/blob/30c8202c630c5af2e3c3403df1bb95c962459bc6/Configuration/UTMAppleConfiguration.swift#L314-L320) as the tag, which seems to have the value `com.apple.virtio-fs.automount`.

Running the following in the guest worked for me:

```
mkdir -m 777 -p ~/my-shared-files
sudo umount "/Volumes/My Shared Files"
sudo mount_virtiofs com.apple.virtio-fs.automount ~/my-shared-files
```

Perhaps the documentation could be updated to integrate this information (perhaps keeping `~` paths to show that those work, and that you don't need the path to be under `/Volumes` as I've seen claimed elsewhere).

Backstory: in my case I need to do this because I want to build some software in the guest that absolutely refuses to build if there are spaces in (normalized) paths.

Thanks for all the work to bring us UTM!

**Configuration**
* UTM Version: 4.7.5 (118)
* macOS Version: 26.4
* Mac Chip: M3