Improve doc example: Extracting the type parameter from a super-type (#…

…55983)

Documentation describes the correct way of extracting the element type
of a supertype:

https://docs.julialang.org/en/v1/manual/methods/#Extracting-the-type-parameter-from-a-super-type

However, one of the examples to showcase this is nonsensical since it is
a union of multiple element types.
I have replaced this example with a union over the dimension.
Now, the `eltype_wrong` function still gives a similar error, yet the
correct way returns the unambiguous answer.

---------

Co-authored-by: Lilith Orion Hafner <[email protected]>

doc/src/manual/methods.md

@@ -698,11 +698,14 @@ While this works for declared types, it fails for types without
 supertypes:
 
 ```julia-repl
-julia> eltype_wrong(Union{AbstractArray{Int}, AbstractArray{Float64}})
-ERROR: MethodError: no method matching supertype(::Type{Union{AbstractArray{Float64,N} where N, AbstractArray{Int64,N} where N}})
+julia> eltype_wrong(Union{Vector{Int}, Matrix{Int}})
+ERROR: MethodError: no method matching supertype(::Type{VecOrMat{Int64}})
+
 Closest candidates are:
-  supertype(::DataType) at operators.jl:43
-  supertype(::UnionAll) at operators.jl:48
+  supertype(::UnionAll)
+   @ Base operators.jl:44
+  supertype(::DataType)
+   @ Base operators.jl:43
 ```
 
 ### Building a similar type with a different type parameter