Update docs to remove reference to legacy token

[duffn]

This updates the documentation reference on legacy tokens. Following the newly linked guide, I was able to generate a new app and token and tested it successfully using both the library and CLI.

$ export SLACK_API_TOKEN=xoxb-mytoken

require 'slack-ruby-client'

Slack.configure do |config|
  config.token = ENV['SLACK_API_TOKEN']
end

client = Slack::Web::Client.new

client.chat_postMessage(channel: '#slack-ruby', text: 'Hello from library')

slack chat postMessage --text="hello from CLI" --channel="#slack-ruby"

Closes #372

[dblock]

Thank you!