Revisit reshaping example possibly outdated by fread recognizing dates

[MichaelChirico]

          Actually this is an interesting point, this example was designed for when `dob_child1` etc. would be read by `fread()` as type `character`, but now they're recognized as dates and read as `IDate`.

Before this example illustrated gender being cast from integer to character, now it only goes integer->integer, but gains the class attribute. Maybe we should revisit this.

Originally posted by @MichaelChirico in #6521 (comment)

[venom1204]

Hi @MichaelChirico, I'd like to work on this issue.

My understanding is that the original example relied on dob columns being read as character to demonstrate a type coercion issue when melted with integer gender columns. Since fread now correctly reads them as IDate, this example is no longer effective.

My plan is to replace the dob_child columns with name_child columns containing character data (e.g., "Anna", "Ben"). This re-introduces the character-integer conflict. When melted, the gender column will be correctly and clearly coerced to character, restoring the vignette's original teaching point. I'll update the surrounding text to reflect the change from dob to name.

If this approach works for you, I will submit a PR with the fix.

[MichaelChirico]

SGTM thanks!