mm: Fix typo in cleancache.txt

Correct spelling "implementatation" to "implementation" in
Documentation/vm/cleancache.txt

Signed-off-by: Masanari Iida <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

Documentation/vm/cleancache.txt

···
 puts		- number of puts attempted (all "succeed")
 flushes		- number of flushes attempted
 
-A backend implementatation may provide additional metrics.
+A backend implementation may provide additional metrics.
 
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