Replaced available_models with available_keys in docs

[sumanmichael]

Replaced available_models in docs/source/general/registry.rst with available_keys

What does this PR do?

Typo in docs/source/general/registry.rst

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[codecov]

Codecov Report

Merging #602 (fe6240d) into master (4cf522e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #602   +/-   ##
=======================================
  Coverage   90.90%   90.90%           
=======================================
  Files         132      132           
  Lines        7795     7795           
=======================================
  Hits         7086     7086           
  Misses        709      709           

Flag	Coverage Δ
gpu	?
pytest	?
unittests	90.90% <ø> (ø)

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[ethanwharris]

Nice! Looks good to me 😃