Fixed link in book, moved from http to https

[stefan-k]

Website and book now work with https. Also addresses #315.

[codecov-commenter]

Codecov Report

Base: 93.39% // Head: 93.36% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (2c02c4b) compared to base (d3a89e9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
- Coverage   93.39%   93.36%   -0.04%     
==========================================
  Files         119      119              
  Lines       19012    18986      -26     
==========================================
- Hits        17756    17726      -30     
- Misses       1256     1260       +4     

Impacted Files	Coverage Δ
argmin/src/core/termination.rs	76.19% <0.00%> (-7.69%)	⬇️
argmin/src/solver/simulatedannealing/mod.rs	85.65% <0.00%> (-0.72%)	⬇️

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