Updated the url for RFC 19 so that it isn't a 404.

[jeffgca]

When I tried to use npm --depth 9999 update based on some old documentation, I got an error message:

npm WARN update The --depth option no longer has any effect. See RFC0019. npm WARN update https://github.com/npm/rfcs/blob/latest/accepted/0019-remove-update-depth-option.md

I noticed that the url mentioned is a 404, so this change updates the url from

https://github.com/npm/rfcs/blob/latest/accepted/0019-remove-update-depth-option.md

to

https://github.com/npm/rfcs/blob/latest/implemented/0019-remove-update-depth-option.md

Aside: these docs on npm-update still seem to recommend --depth tricks:

https://docs.npmjs.com/cli/v7/commands/npm-update