DOC: editing tif url for the zonal statistics example

[baloola]

• Closes DOC: Missing file link in example #91
• Tests added
• Fully documented, including docs/history.rst for all changes and docs/geocube.rst for new API

[codecov]

Codecov Report

Base: 97.45% // Head: 97.13% // Decreases project coverage by -0.32% ⚠️

Coverage data is based on head (466eb10) compared to base (53e1828).
Patch coverage: 77.77% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   97.45%   97.13%   -0.33%     
==========================================
  Files          14       13       -1     
  Lines         433      419      -14     
==========================================
- Hits          422      407      -15     
- Misses         11       12       +1     

Impacted Files	Coverage Δ
geocube/_show_versions.py	93.33% <66.66%> (-1.79%)	⬇️
geocube/cli/geocube.py	100.00% <100.00%> (ø)
geocube/_version.py	0.00% <0.00%> (-100.00%)	⬇️
geocube/__init__.py

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[snowman2]

Thanks @baloola 👍