Fix wrong varname in bucket doc

[zxdawn]

A simple fix of the wrong varname used in bucket doc.

[codecov]

Codecov Report

Merging #334 (2aa3f29) into master (c440d1b) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
- Coverage   92.58%   92.50%   -0.08%     
==========================================
  Files          48       48              
  Lines        9826     9757      -69     
==========================================
- Hits         9097     9026      -71     
- Misses        729      731       +2     

Flag	Coverage Δ
unittests	92.50% <ø> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyresample/bucket/__init__.py	98.95% <ø> (ø)
pyresample/test/test_swath.py	87.23% <0.00%> (-4.26%)	⬇️
pyresample/geometry.py	83.48% <0.00%> (-0.43%)	⬇️
pyresample/utils/cartopy.py	94.33% <0.00%> (-0.40%)	⬇️
pyresample/test/test_plot.py	87.39% <0.00%> (-0.21%)	⬇️
pyresample/ewa/dask_ewa.py	89.31% <0.00%> (-0.14%)	⬇️
pyresample/boundary.py	94.44% <0.00%> (-0.11%)	⬇️
pyresample/bilinear/xarr.py	92.12% <0.00%> (-0.07%)	⬇️
pyresample/test/test_dask_ewa.py	97.50% <0.00%> (-0.07%)	⬇️
pyresample/test/test_gradient.py	97.93% <0.00%> (-0.05%)	⬇️
... and 7 more

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[coveralls]

Coverage remained the same at 92.571% when pulling 2aa3f29 on zxdawn:bucket_doc into c440d1b on pytroll:master.

[pnuu]

LGTM