Update StatsPlots requirement to 0.15.8 in /docs

Updates the requirements on [StatsPlots](https://github.com/JuliaPlots/StatsPlots.jl) to permit the latest version.
- [Release notes](https://github.com/JuliaPlots/StatsPlots.jl/releases)
- [Changelog](https://github.com/JuliaPlots/StatsPlots.jl/blob/master/NEWS.md)
- [Commits](JuliaPlots/StatsPlots.jl@v0.0.1...v0.15.8)

---
updated-dependencies:
- dependency-name: StatsPlots
  dependency-version: 0.15.8
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/Project.toml

@@ -17,3 +17,4 @@ CodecXz = "0.7.4"
 Documenter = "1.16.1"
 Latexify = "0.16.10"
 Plots = "1.41.3"
+StatsPlots = "0.15.8"