Fix type in NNX readme

[shoyer]

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (85dfad2) 56.31% compared to head (1c7ff8b) 56.31%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3591   +/-   ##
=======================================
  Coverage   56.31%   56.31%           
=======================================
  Files         100      100           
  Lines       11952    11952           
=======================================
  Hits         6731     6731           
  Misses       5221     5221           

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[cgarciae]

Thanks @shoyer !