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.
It's kind of awkward that this takes the scheme-relative URL -- either
the full URL, or maybe just the host, would make more sense. But on a
bit of study of the plugin's source, I can't convince myself that it
won't break if given the full URL with the scheme. (See a code
comment thread in #7116.) So leave the substance as is, pending the
plugin itself getting some cleanup, but do fix the sentence's English
grammar since we're looking at it.
The "subdomain" label is redundant, to the extent it's even
accurate -- this is really just the URL we want to display,
which may or may not involve a subdomain. Similarly "external".
The former `external_api_path_subdomain` was never a path -- it's a
host, followed by a path, which together form a scheme-relative URL.
I'm not quite convinced that value is actually the right thing in
2 of the 3 places we use it, but fixing that can start by giving an
accurate name to the thing we have.