Ignore slack archives link (Qiskit#1481)

Closes Qiskit#1478

The link checker complains about a Slack link that is not broken. This
PR adds the link to the ignores.

scripts/lib/links/ignores.ts

@@ -35,6 +35,7 @@ const ALWAYS_IGNORED_URLS__EXPECTED = [
   "https://auth.quantum-computing.ibm.com/api",
   "https://www.cs.tau.ac.il/~nogaa/PDFS/r.pdf",
   "http://www.satcompetition.org/2009/format-benchmarks2009.html",
+  "https://qiskit.slack.com/archives/C06KF8YHUAU",
   // StackOverflow rate limits us.
   "https://stackoverflow.com/",
   "https://stackoverflow.com/questions/1049722/what-is-2s-complement",