Update experiments-covid.md

[lintool]

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[codecov]

Codecov Report

Merging #1071 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1071      +/-   ##
============================================
- Coverage     44.76%   44.75%   -0.02%     
+ Complexity      650      649       -1     
============================================
  Files           135      135              
  Lines          8040     8040              
  Branches       1149     1149              
============================================
- Hits           3599     3598       -1     
  Misses         4118     4118              
- Partials        323      324       +1     

Impacted Files	Coverage Δ	Complexity Δ
...anserini/ltr/feature/base/PMIFeatureExtractor.java	84.61% <0.00%> (-1.93%)	12.00% <0.00%> (-1.00%)

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