Fix docs

[schneems]

https://github.com/heroku/buildpacks-ruby/blob/8491520a1b91338127e31de94353e8ceb9f6a337/docs/upgrading.md

Failure to detect rake tasks will fail a deployment. On all builds rake -p is called to find a list of all rake tasks. If this detection task fails then the previous behavior was to fail the build only if the sprockets gem was present. The reason was to allow API-only apps that don't need to generate assets to have a Rakefile that cannot be run in production (the most common reason is they're requiring a library not in their production gemfile group). Now all failures when loading a Rakefile will fail the build.

It should read "railties" rather than sprockets https://github.com/heroku/heroku-buildpack-ruby/blob/abd7281af5734358c2144cd5b4267af4728336b8/lib/language_pack/ruby.rb#L905-L909. However sprockets seems more general and perhaps a better gem to test against. It could be sprockets or rails (as newer versions of rails no longer use sprockets).