[docs] fix thread safefy issue in parallelism tutorial

[odow]

As pointed out on Discourse, https://discourse.julialang.org/t/jump-multi-threading-example/95210/2, I didn't check carefully enough and I assumed that push! was an atomic operation in Julia.

[codecov]

Codecov Report

Base: 98.07% // Head: 98.08% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (c69b2a5) compared to base (385c72f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3240   +/-   ##
=======================================
  Coverage   98.07%   98.08%           
=======================================
  Files          34       34           
  Lines        4632     4656   +24     
=======================================
+ Hits         4543     4567   +24     
  Misses         89       89           

Impacted Files	Coverage Δ
src/JuMP.jl	96.82% <ø> (ø)
src/optimizer_interface.jl	96.33% <100.00%> (+0.45%)	⬆️

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[ericphanson]

I don’t think this is safe either, xref JuliaLang/julia#48542

[odow]

Urgh. Okay. I've updated to use a lock, which is what I probably should have used right from the start.