Updated guide for MS MARCO doc retrieval

[lintool]

@edwinzhng wasn't able to replicate, and I confirmed that the documentation is incorrect.

@edwinzhng can you match your numbers with these? give me a +1 if they match - thanks!

[codecov]

Codecov Report

Merging #949 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #949      +/-   ##
============================================
- Coverage     39.55%   39.48%   -0.07%     
+ Complexity      515      511       -4     
============================================
  Files           120      120              
  Lines          7231     7231              
  Branches       1086     1086              
============================================
- Hits           2860     2855       -5     
- Misses         4083     4088       +5     
  Partials        288      288

Impacted Files	Coverage Δ	Complexity Δ
...java/io/anserini/ltr/feature/CountBigramPairs.java	83.11% <0%> (-6.5%)	29% <0%> (-4%)

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[edwinzhng]

👍 was able to replicate the same values