fix: return `str` instead of `list` when executing `tldr -l`

[uunnxx]

Return str separated by \n.

It helps when using by other programs, like fzf I've alias: alias ch="tldr \$(tldr -l | fzf)" so it'll search through all pages.

Closes #221.

[vitorhcl]

@kbdharun @acuteenvy as this is a minor change, I think you can apply @acuteenvy's suggestion and merge this