doc: example region extraction in X-ray fits fails

[Xarthisius]

Bug report

Bug summary

See: https://yt-project.org/doc/cookbook/fits_xray_images.html

Code for reproduction

from yt.frontends.fits.misc import ds9_region
import yt

ds = yt.load(
    "xray_fits/A2052_merged_0.3-2_match-core_tmap_bgecorr.fits",
    auxiliary_files=["xray_fits/A2052_core_tmap_b1_m2000_.fits"],
)

exposure_time = ds.quan(ds.primary_header["exposure"], "s")
reg_file = [
    "# Region file format: DS9 version 4.1\n",
    "global color=green dashlist=8 3 width=3 include=1 source=1 fk5\n",
    'circle(15:16:44.817,+7:01:19.62,34.6256")',
]
f = open("circle.reg", "w")
f.writelines(reg_file)
f.close()
circle_reg = ds9_region(
    ds, "circle.reg", field_parameters={"exposure_time": exposure_time}
)

Actual outcome

WARNING: A coordinate frame was not found for region: "circle(15:16:44.817,+7:01:19.62,34.6256")", skipping. [regions.io.ds9.read]
Traceback (most recent call last):
  File "/home/xarth/yt/xray_fits/ala.py", line 18, in <module>
    circle_reg = ds9_region(
  File "/home/xarth/codes/xarthisius/yt/yt/frontends/fits/misc.py", line 172, in ds9_region
    r = method(reg, format="ds9").regions[0]
IndexError: list index out of range

Expected outcome

No error.