Update OptimizationNLopt requirement to 0.3.9 in /docs

Updates the requirements on [OptimizationNLopt](https://github.com/SciML/Optimization.jl) to permit the latest version.
- [Release notes](https://github.com/SciML/Optimization.jl/releases)
- [Changelog](https://github.com/SciML/Optimization.jl/blob/master/NEWS.md)
- [Commits](https://github.com/SciML/Optimization.jl/commits)

---
updated-dependencies:
- dependency-name: OptimizationNLopt
  dependency-version: 0.3.9
  dependency-type: direct:production
...

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Author: dependabot[bot]

docs/Project.toml

@@ -28,6 +28,7 @@ DFTK = "0.7"
 Documenter = "~1.8, 1.16"
 EmpiricalPotentials = "0.2.5"
 GeometryOptimization = "0.1.5"
+OptimizationNLopt = "0.3.9"
 PseudoPotentialData = "0.2"
 Unitful = "1.28.0"
 UnitfulAtomic = "1.0.0"