Fix doctest output labelling

[mortenpi]

I find the current labeling of the outputs when printing a bit confusing. Currently it looks like:

┌ Error: doctest failure in src/showcase.md:17-20
│ 
│ ```jldoctest
│ julia> 2 + 2
│ 5
│ ```
│ 
│ Subexpression:
│ 
│ 2 + 2
│ 
│ Output:
│ 
│ 5
│ 
│ Expected output:
│ 
│ 4

I think "Expected output" should refer to the original doctest, since we expect the doctested code to match the docs. So this PR swaps them around and instead says:

┌ Error: doctest failure in src/showcase.md:17-20
│ 
│ ```jldoctest
│ julia> 2 + 2
│ 5
│ ```
│ 
│ Subexpression:
│ 
│ 2 + 2
│ 
│ Evaluated output:
│ 
│ 4
│ 
│ Expected output:
│ 
│ 5