Unused generic in type-safety.md

[Meemaw]

There is a unused generic in the https://github.com/aralroca/next-translate/blob/master/docs/type-safety.md.

<T extends string>.

[aralroca]

thanks to report it @Meemaw !

we added this part to support template literals:

t`some-key`

In template literals is not possible yet to do type safety, but we did this to allow strings. But not sure if this generic is necessary, maybe we can remove it! Feel free contribute and do a PR fixing this 👍