xfsdocs: fix a minor levelling problem

Fix these incorrect heading levels.

asciidoc: WARNING: allocation_groups.asciidoc: line 1089: section title out of sequence: expected level 3, got level 4
asciidoc: WARNING: allocation_groups.asciidoc: line 1218: section title out of sequence: expected level 3, got level 4

Signed-off-by: Darrick J. Wong <djwong@kernel.org>

design/XFS_Filesystem_Structure/allocation_groups.asciidoc

diff --git a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
index b088abb..0e48b4b 100644
--- a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
+++ b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
@@ -1077,7 +1077,7 @@
 image::images/20b.png[]
 
 
-==== xfs_db AGI Example
+=== xfs_db AGI Example
 
 This is an AGI of a freshly populated filesystem:
 
@@ -1206,7 +1206,7 @@
 A 64 element bitmap showing which inodes in this chunk are not available for
 allocation.
 
-==== xfs_db Sparse Inode AGI Example
+=== xfs_db Sparse Inode AGI Example
 
 This example derives from an AG that has been deliberately fragmented.  The
 inode B+tree: