docs: admin-guide: Fix typo in nfsroot.rst

There is an obvious mistake in nfsroot.rst where pxelinux was wrongly
written as pxeliunx. Fix it.

Signed-off-by: Zenghui Yu <zenghui.yu@linux.dev>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250827144738.43361-1-zenghui.yu@linux.dev