Fix typo in running-locust-in-step-load-mode.rst

[sgajjar]

Fix typo

[codecov]

Codecov Report

Merging #1298 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1298   +/-   ##
=======================================
  Coverage   79.60%   79.60%           
=======================================
  Files          23       23           
  Lines        2079     2079           
  Branches      323      323           
=======================================
  Hits         1655     1655           
  Misses        342      342           
  Partials       82       82           

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