doc: fix documentation of enum gd_flags

Correct GD_FLG_CYCLIC_RUNNING documentation to match Sphinx style.

Fixes: d7de5ef6 ("cyclic: use a flag in gd->flags for recursion protection")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>