docs: cgroup: fixed spelling mistakes in documentation

found/fixed the following typo

- Availablity -> Availability

in `Documentation/admin-guide/cgroup-v2.rst`

Signed-off-by: Soham Metha <sohammetha01@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Soham-Metha

Documentation/admin-guide/cgroup-v2.rst

@@ -435,8 +435,8 @@ both cgroups.
 Controlling Controllers
 -----------------------
 
-Availablity
-~~~~~~~~~~~
+Availability
+~~~~~~~~~~~~
 
 A controller is available in a cgroup when it is supported by the kernel (i.e.,
 compiled in, not disabled and not attached to a v1 hierarchy) and listed in the