docs: rename Documentation/vm to Documentation/mm

so it will be consistent with code mm directory and with
Documentation/admin-guide/mm and won't be confused with virtual machines.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Tested-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Wu XiangCheng <bobwxc@email.cn>

Documentation/admin-guide/sysctl/vm.rst

@@ -760,7 +760,7 @@ and don't use much of it.
 
 The default value is 0.
 
-See Documentation/vm/overcommit-accounting.rst and
+See Documentation/mm/overcommit-accounting.rst and
 mm/util.c::__vm_enough_memory() for more information.