fixes reference to old and missing vignette in get_token, get_tokens

[dshkol]

Previously the documentation in tokens.r showed this:

...
#' to be instantly [re]loaded in future sessions. See the "tokens"
#' vignette for instructions on obtaining and using access tokens.
...

Which refers a vignette that has been replaced by one named "auth" as reference in other .R documentation files. Found this confusing when trying to get help from ?get_tokens and couldn't find the vignette from my rsession and had to navigate to the website to figure out the vignette didn't exist.

Updated tokens.r, and generated get_tokens.rd and .../get_tokens.html by calling devtools:document() and pkgdown::build_articles

[llrs]

Thanks!