docs: Add two missing entries in vm sysctl index

Both seem overlooked while adding the section in the main content.

Signed-off-by: Fam Zheng <famzheng@amazon.com>
Link: https://lore.kernel.org/r/20201022065403.3936070-1-fam@euphon.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: Fam Zheng

Documentation/admin-guide/sysctl/vm.rst

@@ -27,6 +27,7 @@ Currently, these files are in /proc/sys/vm:
 - admin_reserve_kbytes
 - block_dump
 - compact_memory
+- compaction_proactiveness
 - compact_unevictable_allowed
 - dirty_background_bytes
 - dirty_background_ratio
@@ -37,6 +38,7 @@ Currently, these files are in /proc/sys/vm:
 - dirty_writeback_centisecs
 - drop_caches
 - extfrag_threshold
+- highmem_is_dirtyable
 - hugetlb_shm_group
 - laptop_mode
 - legacy_va_layout