doc: typo in Documentation/filesystems/nfsroot.txt

Add a missing word to the explanation of the purpose of the zdisk and
bzdisk make targets.

Signed-off-by: Shane McDonald <mcdonald.shane@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>