Rename SatPy to Satpy throughout documentation

[djhoese]

As discussed on slack, from now on we will try to standardize Pytroll package names in text by capitalizing the first letter and no other letters unless there is an acronym. This means Pyresample, Trollimage, Pytroll (the organization), and Satpy.

Muscle memory is going to be a problem...

[codecov]

Codecov Report

Merging #699 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #699   +/-   ##
=======================================
  Coverage   79.31%   79.31%           
=======================================
  Files         145      145           
  Lines       21120    21120           
=======================================
  Hits        16752    16752           
  Misses       4368     4368

Impacted Files	Coverage Δ
.../tests/reader_tests/test_viirs_edr_active_fires.py	96.18% <ø> (ø)	⬆️
satpy/readers/generic_image.py	94.36% <ø> (ø)	⬆️
satpy/tests/test_demo.py	93.84% <ø> (ø)	⬆️
satpy/readers/virr_l1b.py	98.27% <ø> (ø)	⬆️
satpy/demo/google_cloud_platform.py	95% <ø> (ø)	⬆️
satpy/readers/abi_l1b.py	95.9% <ø> (ø)	⬆️
satpy/readers/__init__.py	94.87% <ø> (ø)	⬆️
satpy/readers/viirs_edr_active_fires.py	90% <ø> (ø)	⬆️
satpy/demo/__init__.py	95.45% <ø> (ø)	⬆️
satpy/tests/test_dataset.py	97.72% <ø> (ø)	⬆️
... and 8 more

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[coveralls]

Coverage remained the same at 79.318% when pulling 95fafd8 on djhoese:doc-satpy-capitals into 9132476 on pytroll:master.