Update MetaGraphs requirement to 0.8.1 in /docs

Updates the requirements on [MetaGraphs](https://github.com/JuliaGraphs/MetaGraphs.jl) to permit the latest version.
- [Release notes](https://github.com/JuliaGraphs/MetaGraphs.jl/releases)
- [Commits](v0.7.1...v0.8.1)

---
updated-dependencies:
- dependency-name: MetaGraphs
  dependency-version: 0.8.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

docs/Project.toml

@@ -4,3 +4,4 @@ MetaGraphs = "626554b9-1ddb-594c-aa3c-2596fe9399a5"
 
 [compat]
 Documenter = "0.27"
+MetaGraphs = "0.8.1"