btrfs-progs: docs: fix doc build issue caused by confusion between BTRFS_ and target

Text ending with `_` is treated as an anonymous hyperlink. Use an escape
character `\` to prevent this.

Making all in Documentation
    [SPHINX] man
btrfs-progs/Documentation/dev/On-disk-format.rst:32: ERROR: Unknown target name: "btrfs".

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: asj

Documentation/dev/On-disk-format.rst

@@ -30,7 +30,7 @@ also contains a logical reference to root nodes in the root and chunk trees,
 which can then be used to locate all the other trees and data stored.
 
 To avoid duplicated suffixes/prefixes, sometimes the macro name will have
-the "BTRFS_" prefix and "_OBJECTID" suffix removed.
+the "BTRFS\_" prefix and "_OBJECTID" suffix removed.
 
 E.g. "BTRFS_DEV_ITEMS_OBJECTID" (0x1) can be shown as "DEV_ITEMS" for short,
 this matches the output of "btrfs inspect-internal dump-tree".