Fix 'd rop-in' typo to 'drop-in'

[jenglamlow]

No description provided.

[codecov]

Codecov Report

Merging #1215 into master will decrease coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1215      +/-   ##
=========================================
- Coverage    78.8%   78.6%   -0.21%     
=========================================
  Files          20      20              
  Lines        1958    1958              
  Branches      308     308              
=========================================
- Hits         1543    1539       -4     
- Misses        336     339       +3     
- Partials       79      80       +1

Impacted Files	Coverage Δ
locust/runners.py	78.33% <0%> (-1.01%)	⬇️

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