qemu-img-cmds.hx: Update comment that mentions Texinfo

Missed in 3c95fdef94 "Update comments in .hx files that mention
Texinfo".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200929075824.1517969-2-armbru@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

qemu-img-cmds.hx

@@ -1,5 +1,5 @@
 HXCOMM Keep the list of subcommands sorted by name.
-HXCOMM Use DEFHEADING() to define headings in both help text and texi
+HXCOMM Use DEFHEADING() to define headings in both help text and rST
 HXCOMM Text between SRST and ERST are copied to rST version and
 HXCOMM discarded from C version
 HXCOMM DEF(command, callback, arg_string) is used to construct