bug 1560497: remote: use public Slack instance URL; r=me a=doc

There’s a new signup page for the Slack instance at
https://devtools-html-slack.herokuapp.com/ which accepts non-MoCo
email addresses. We should use this in our public documentation so
contributors can sign in.

DONTBUILD

remote/doc/index.rst

@@ -59,4 +59,4 @@ the *#remote* channel on the `DevTools Slack`_ instance.
 .. _dev-remote@lists.mozilla.org: mailto:dev-remote@lists.mozilla.org
 .. _subscribe: https://lists.mozilla.org/listinfo/dev-remote
 .. _archive: https://lists.mozilla.org/pipermail/dev-remote/
-.. _DevTools Slack: https://devtools-html.slack.com/
+.. _DevTools Slack: https://devtools-html-slack.herokuapp.com/