fix twisted link in README.md

[wbarnha]

I forgot to fix the twisted link in my last PR.

[codecov-commenter]

Codecov Report

Merging #309 (5ad9fca) into master (c4f5b18) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #309   +/-   ##
=======================================
  Coverage   94.66%   94.66%           
=======================================
  Files         100      100           
  Lines       10855    10855           
  Branches     1520     1520           
=======================================
  Hits        10276    10276           
  Misses        505      505           
  Partials       74       74           

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