Update experiments-covid.md

[lintool]

Oops, sorry, broke previous instructions.

[lintool]

Let me run through all instructions again before I merge...

[nikhilro]

Coolio, haha

[nikhilro]

The second and third version of the Collection won't work right now. link

[codecov]

Codecov Report

Merging #1052 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1052      +/-   ##
============================================
+ Coverage     43.01%   43.06%   +0.04%     
- Complexity      635      636       +1     
============================================
  Files           132      132              
  Lines          8083     8083              
  Branches       1168     1168              
============================================
+ Hits           3477     3481       +4     
+ Misses         4276     4272       -4     
  Partials        330      330              

Impacted Files	Coverage Δ	Complexity Δ
...anserini/ltr/feature/base/PMIFeatureExtractor.java	86.53% <0.00%> (+1.92%)	13.00% <0.00%> (+1.00%)
...java/io/anserini/ltr/feature/CountBigramPairs.java	93.50% <0.00%> (+3.89%)	33.00% <0.00%> (ø%)

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