Documentation: Fix grammatical error in sysctl/fs.txt & clarify negative dentry

Fix a grammatical error in the dentry-state text and clarify the usage
of negative dentries.

Fixes: af0c9af1

 ("fs/dcache: Track & report number of negative dentries")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>