Removes the three remaining instances of `settings_html` links in
the integrations documentation. Use of `settings_html` was removed
from the general shared `create-an-incoming-webhook.md` and
`create-a-generic-bot.md` files in e9e2721.
An integration with RSS can be much easier to set up with Zapier.
Since the current RSS integration predates Zapier, we should at
least mention that there is an easier way to set it up via
Zapier!