Documentation: Fix typos found in cgroup-v2.rst

Fix typos found in Documentation/admin-guide/cgroup-v2.rst.
Signed-off-by: NJiang Biao <benbjiang@tencent.com>
Link: https://lore.kernel.org/r/20210107141118.9530-1-benbjiang@tencent.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/cgroup-v2.rst

@@ -1558,7 +1558,7 @@ IO Interface Files
 	  8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021
 
   io.cost.qos
-	A read-write nested-keyed file with exists only on the root
+	A read-write nested-keyed file which exists only on the root
 	cgroup.
 
 	This file configures the Quality of Service of the IO cost
@@ -1613,7 +1613,7 @@ IO Interface Files
 	automatic mode can be restored by setting "ctrl" to "auto".
 
   io.cost.model
-	A read-write nested-keyed file with exists only on the root
+	A read-write nested-keyed file which exists only on the root
 	cgroup.
 
 	This file configures the cost model of the IO cost model based