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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/admin-guide/nfs/nfsroot.rst

···
 	When using pxelinux, the kernel image is specified using
 	"kernel <relative-path-below /tftpboot>". The nfsroot parameters
 	are passed to the kernel by adding them to the "append" line.
-	It is common to use serial console in conjunction with pxeliunx,
+	It is common to use serial console in conjunction with pxelinux,
 	see Documentation/admin-guide/serial-console.rst for more information.
 
 	For more information on isolinux, including how to create bootdisks