doc: Fix a typo in slub.txt.

Signed-off-by: Justin P. Mattock <justinmattock@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/vm/slub.txt

@@ -245,7 +245,7 @@ been overwritten. Here a string of 8 characters was written into a slab that
 has the length of 8 characters. However, a 8 character string needs a
 terminating 0. That zero has overwritten the first byte of the Redzone field.
 After reporting the details of the issue encountered the FIX SLUB message
-tell us that SLUB has restored the Redzone to its proper value and then
+tells us that SLUB has restored the Redzone to its proper value and then
 system operations continue.
 
 Emergency operations: