Fix missing link in mixed.md

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5043ecf) 95.19% compared to head (bdd3254) 95.19%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #213 +/- ## ======================================= Coverage 95.19% 95.19% ======================================= Files 14 14 Lines 1644 1644 ======================================= Hits 1565 1565 Misses 79 79 ```

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Package name	latest	stable
CaNNOLeS.jl
DCISolver.jl
DerivativeFreeSolvers.jl
JSOSolvers.jl
NLPModelsIpopt.jl
OptimizationProblems.jl
Percival.jl
QuadraticModels.jl
SolverBenchmark.jl
SolverTools.jl