Generator spec reference link is outdated in `runtime.js` file

[rbals0445]

Hello! (related : https://github.com/facebook/regenerator/issues/158)

While looking at the regenerator code, I found that the link was not working properly.

https://github.com/facebook/regenerator/blob/main/packages/runtime/runtime.js#L263

I think you meant number three of GeneratorResume. so it would be nice to replace the link with an official document

https://262.ecma-international.org/6.0/#sec-generatorresume

thank you!