Update NLsolve requirement to 4.5.1 in /docs (#197)

Updates the requirements on [NLsolve](https://github.com/JuliaNLSolvers/NLsolve.jl) to permit the latest version.
- [Release notes](https://github.com/JuliaNLSolvers/NLsolve.jl/releases)
- [Commits](JuliaNLSolvers/NLsolve.jl@v0.1.0...v4.5.1)

---
updated-dependencies:
- dependency-name: NLsolve
  dependency-version: 4.5.1
  dependency-type: direct:production
...

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docs/Project.toml

@@ -19,3 +19,4 @@ ADTypes = "1.21.0"
 Documenter = "1.3"
 ForwardDiff = "1.3.1"
 Literate = "2.21.0"
+NLsolve = "4.5.1"