docs: cgroup-v1: correct the term of Page Cache organization in inode

The radix-tree for Page Cache has been replaced with xarray, see
commit eb797a8e ("page cache: Rearrange address_space"), so move
"radix-tree" to "xarray".

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Signed-off-by: Tejun Heo <tj@kernel.org>