Fix typo: fix badly conjugated verb in typescript roadmap 108-enum.md

[Malv93]

Fix a typo with a third person verb badly conjugated. I added an "s" at the end of "It allow".

[Malv93]

Hi @kamranahmedse, I see that you are the repo owner, so I am tagging you.

I'm sorry to bother you, but I'd like to start helping and contributing to this roadmap that I found so much helpful to start learning software programming.

This is a very simple PR to fix a typo I found in a roadmap file. I did not open an issue, following the repo guide, since it's a very simple typo.
Who can review this PR?
Maybe a simple guide about who is the owner/maintainer of some part of the project would be helpful to send PRs to the right people.

I'm also tagging @arikchakma, since I see you are a very active maintainer.

Thank you guys, and have a nice day ;)