Fixed some spelling/grammar on docstrings in core.py

[Trouv]

New here, making a small change to some docs

[codecov]

Codecov Report

Merging #1323 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1323      +/-   ##
==========================================
- Coverage   80.98%   80.89%   -0.10%     
==========================================
  Files          24       24              
  Lines        2198     2198              
  Branches      332      332              
==========================================
- Hits         1780     1778       -2     
- Misses        333      334       +1     
- Partials       85       86       +1     

Impacted Files	Coverage Δ
locust/core.py	97.71% <ø> (ø)
locust/web.py	89.65% <0.00%> (-1.15%)	⬇️

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[heyman]

Thanks!