Update Optim requirement to 2.0.0 in /docs

Updates the requirements on [Optim](https://github.com/JuliaNLSolvers/Optim.jl) to permit the latest version.
- [Release notes](https://github.com/JuliaNLSolvers/Optim.jl/releases)
- [Changelog](https://github.com/JuliaNLSolvers/Optim.jl/blob/master/NEWS.md)
- [Commits](JuliaNLSolvers/Optim.jl@v0.2.0...v2.0.0)

---
updated-dependencies:
- dependency-name: Optim
  dependency-version: 2.0.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/Project.toml

@@ -21,3 +21,4 @@ Documenter = "1.3"
 ForwardDiff = "1.3.1"
 Literate = "2.21.0"
 NLsolve = "4.5.1"
+Optim = "2.0.0"