Documentation: cgroups: just fix a few typos

Just fix some typos in blkio-controller.txt, freezer-subsystem.txt,
unified-hierarchy.txt.

Signed-off-by: Yuan Sun <sunyuan3@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/cgroups/freezer-subsystem.txt b/Documentation/cgroups/freezer-subsystem.txt
index c96a72c..e831cb2 100644
--- a/Documentation/cgroups/freezer-subsystem.txt
+++ b/Documentation/cgroups/freezer-subsystem.txt
@@ -50,7 +50,7 @@
 expected.
 
 The cgroup freezer is hierarchical. Freezing a cgroup freezes all
-tasks beloning to the cgroup and all its descendant cgroups. Each
+tasks belonging to the cgroup and all its descendant cgroups. Each
 cgroup has its own state (self-state) and the state inherited from the
 parent (parent-state). Iff both states are THAWED, the cgroup is
 THAWED.