README: fix reference to open_tree kernel requirements

open_tree(2) and fsopen(2) were added in Linux 5.2, not 4.18.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

README.md

@@ -86,7 +86,7 @@ more secure. In particular:
   or avoid being tricked by a `/proc` that is not legitimate. This is done
   using [`openat2`][openat2.2] for all users, and privileged users will also be
   further protected by using [`fsopen`][fsopen.2] and [`open_tree`][open_tree.2]
-  (Linux 4.18 or later).
+  (Linux 5.2 or later).
 
 [openat2.2]: https://www.man7.org/linux/man-pages/man2/openat2.2.html
 [fsopen.2]: https://github.com/brauner/man-pages-md/blob/main/fsopen.md