change base url in POOCH for data retrieval

[SarthakJariwala]

Changing the base url fixed the HTTPError that pooch was running into while downloading data

[codecov-io]

Codecov Report

Merging #157 (bf04736) into master (a573ced) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #157   +/-   ##
=======================================
  Coverage   95.14%   95.14%           
=======================================
  Files           9        9           
  Lines         700      700           
  Branches      170      170           
=======================================
  Hits          666      666           
  Misses         17       17           
  Partials       17       17           

Impacted Files	Coverage Δ
src/seaborn_image/_datasets.py	100.00% <ø> (ø)

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