Fix a typo in Documentation/keys.txt

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>