xfsdocs: Small fix to correct first free inode to be 5847 not 5856.

Fix a minor math error in the demonstration of inode btrees.

Signed-off-by: Lukas Herbolt <lukas@herbolt.com>
[djwong: tweak the commit message a bit]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
diff --git a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
index 992615d..cdc8545 100644
--- a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
+++ b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
@@ -1099,7 +1099,7 @@
 Most of the inode chunks on this filesystem are totally full, since the +free+
 value is zero.  This means that we ought to expect inode 160 to be linked
 somewhere in the directory structure.  However, notice that 0xff80000000000000
-in record 85 -- this means that we would expect inode 5856 to be free.  Moving
+in record 85 -- this means that we would expect inode 5847 to be free.  Moving
 on to the free inode B+tree, we see that this is indeed the case: