fix reference to single browse image in insar_tops_burst readme

[asjohnston-asf]

We only have a single browse image for burst insar products at the moment, created from the unwrapped phase geotiff.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
/home/runner/micromamba-root/envs/hyp3-isce2/lib/python3.11/site-packages/hyp3_isce2
__main__.py	26	2	92%	33, 44
burst.py	146	51	65%	78–81, 94–99, 113–128, 144–153, 167–177, 235, 282–290, 310–324
insar_stripmap.py	34	13	62%	40, 57–75
insar_tops.py	56	28	50%	51–84, 105–125
insar_tops_burst.py	189	145	23%	69–113, 125–150, 179–256, 267–349, 361–362, 390–466
logging.py	4	1	75%	9
s1_auxcal.py	21	2	90%	51, 59
slc.py	34	10	71%	22–25, 29–34
topsapp.py	69	8	88%	74, 104, 126–132
utils.py	29	3	90%	66–68
/home/runner/micromamba-root/envs/hyp3-isce2/lib/python3.11/site-packages/hyp3_isce2/metadata
util.py	14	9	36%	6–14, 18–21, 25–27
TOTAL	695	272	61%

Tests	Skipped	Failures	Errors	Time
34	0 💤	0 ❌	0 🔥	1m 28s ⏱️