fix docs typo in client.md

[haakon-e]

HTTP.Pool(; max::Int) -> HTTP.Pool(max::Int)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.19%. Comparing base (e27493d) to head (b08580b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1160      +/-   ##
==========================================
+ Coverage   82.62%   83.19%   +0.56%     
==========================================
  Files          32       32              
  Lines        3051     3053       +2     
==========================================
+ Hits         2521     2540      +19     
+ Misses        530      513      -17     

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[quinnj]

Thanks!