Fix the example in parseutils and make it auto-checking

[kaushalmodi]

Reviving the PR originally created by @albmoron in #13227. If the OP brings back their PR, this can be closed.

/cc @narimiran

[kaushalmodi]

@narimiran @albmoron

Also I was saying is that you can have auto-checking examples in doc strings and have the docs look pretty.

This PR renders this:

[timotheecour]

regarding @narimiran 's comment #13227 (comment) :
commit be8d685 was actually runnning against
#9227 ; if we fix this issue, then we can use runnableExamples pretty much everywhere a doc string was previously preffered

[narimiran]

Although this PR might be objectively better than #13231, we must also take into an account the social aspect - that PR is their first-time contribution to the Nim codebase, and I would feel bad rejecting it based on some nit-picking.