fix(types): fix return type of youtubeDl function

[simonegosetto]

for issue #210

[kylerichter]

This breaks TypeScript projects expecting only Payload but was released as a patch?

[simonegosetto]

hello @kylerichter, there is at least one case where the function in question returns a string and not a Payload, so the Typescript types have simply been aligned, if your code gives an error just cast the result to the right type you expect