Fix simple typo: betwen -> between

[timgates42]

Closes #1268

[codecov]

Codecov Report

Merging #1269 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1269      +/-   ##
==========================================
+ Coverage   77.46%   77.52%   +0.05%     
==========================================
  Files          20       20              
  Lines        1984     1984              
  Branches      313      313              
==========================================
+ Hits         1537     1538       +1     
+ Misses        368      367       -1     
  Partials       79       79

Impacted Files	Coverage Δ
locust/core.py	92.08% <ø> (+0.41%)	⬆️
locust/web.py	87.5% <0%> (ø)	⬆️
locust/rpc/zmqrpc.py	92.1% <0%> (ø)	⬆️
locust/runners.py	72.62% <0%> (ø)	⬆️
locust/stats.py	85.71% <0%> (ø)	⬆️
locust/contrib/fasthttp.py	87.2% <0%> (ø)	⬆️
locust/rpc/protocol.py	100% <0%> (ø)	⬆️

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[cyberw]

Thanks!