docs: Fix image builder instructions.
The `image_builder.sh` script must be run as `root`.
Fixes
@@ -11,7 +11,7 @@ This uses a rootfs directory created by the `rootfs-builder/rootfs.sh` script.
To create a guest OS image run:
```
-$ ./image_builder.sh path/to/rootfs
+$ sudo ./image_builder.sh path/to/rootfs
```
Where `path/to/rootfs` is the directory populated by `rootfs.sh`.
