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: af0c9af1b3 ("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>

Documentation/sysctl/fs.txt

@@ -80,7 +80,9 @@ nonzero when shrink_dcache_pages() has been called and the
 dcache isn't pruned yet.
 
 nr_negative shows the number of unused dentries that are also
-negative dentries which do not mapped to actual files.
+negative dentries which do not map to any files. Instead,
+they help speeding up rejection of non-existing files provided
+by the users.
 
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