Change README docs link to point to stable rather than latest

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #625 (2b1bd2d) into master (4933972) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #625   +/-   ##
=======================================
  Coverage   86.40%   86.40%           
=======================================
  Files         149      149           
  Lines        8458     8458           
=======================================
  Hits         7308     7308           
  Misses       1150     1150           

Flag	Coverage Δ
unittests	86.40% <ø> (ø)

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