[ADAM-2127] Update python doc per GenomicRdd --> GenomicDataset change

[heuermh]

Fixes #2127

[akmorrow13]

This LGTM. Also you were right @heuermh , this will need to be updated on pypi for the changes on readthedocs to be updated. I was looking into this issue last week, but it looks like all the API links on readthedocs are broken . They are incorrectly linking to a folder (overview/) when they should link to api/python.html. I have not been able to replicate this issue on my local computer.

[heuermh]

Bummer. We can merge this and #2126 then, and if/when we come up with a fix for #2131, push the 0.26.0a0 release to PyPI.

[AmplabJenkins]

Test PASSed.
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[coveralls]

Coverage remained the same at 79.075% when pulling 58cd636 on heuermh:issue-2127 into c3324d2 on bigdatagenomics:master.

[heuermh]

Thank you for the review, @akmorrow13