Fix docs problem

[schillic]

The first commit has a change that was automatically applied when I built the docs.

For the second commit, I got the following errors:

┌ Warning: failed to run `@example` block in src/tutorials/set_representations/zonotopes.md:32-36
│ ```@example zonotope_definition
│ @show center(Z)
│ 
│ @show genmat(Z)
│ ```
│   c.value = UndefVarError: center not defined
└ @ Documenter.Expanders ~/.julia/packages/Documenter/qdbx6/src/Expanders.jl:591
┌ Warning: failed to run `@example` block in src/tutorials/set_representations/zonotopes.md:40-42
│ ```@example
│ Z3 = Zonotope([1.0, 0.0], [0.1 0.0; 0.0 0.1])
│ ```
│   c.value = UndefVarError: Zonotope not defined
└ @ Documenter.Expanders ~/.julia/packages/Documenter/qdbx6/src/Expanders.jl:591
┌ Warning: failed to run `@example` block in src/tutorials/set_representations/zonotopes.md:50-55
│ ```@example zonotope_example_1
│ 
│ Z = Zonotope([1, 1.], [-1 0.3 1.5 0.3; 0 0.1 -0.3 0.3])
│ plot(Z)
│ quiver!(fill(1., 4), fill(1., 4), quiver=(genmat(Z)[1, :], genmat(Z)[2, :]), color=:black)
│ ```
│   c.value = UndefVarError: Zonotope not defined
``