docs: add hint of probing loop module
If `loop` module is not probed, it causes error like "losetup: cannot find an unused loop device".
Fixes:
@@ -292,6 +292,7 @@ $ popd
> variable in the previous command and ensure the `qemu-img` command is
> available on your system.
> - If `qemu-img` is not installed, you will likely see errors such as `ERROR: File /dev/loop19p1 is not a block device` and `losetup: /tmp/tmp.bHz11oY851: Warning: file is smaller than 512 bytes; the loop device may be useless or invisible for system tools`. These can be mitigated by installing the `qemu-img` command (available in the `qemu-img` package on Fedora or the `qemu-utils` package on Debian).
+> - If `loop` module is not probed, you will likely see errors such as `losetup: cannot find an unused loop device`. Execute `modprobe loop` could resolve it.
### Install the rootfs image
