Update v5.5.1.md
* If you are using LetsEncrypt, you will want to upgrade to this version because
it includes a fix for a
[bug](https://github.com/concourse/concourse/issues/4326) where updating
-existing acme/autocert/acme certificates fails #4395.
+existing acme/autocert certificates fails #4395.
tldr: upsert autocert certs.
#### <sub><sup><a name="v551-note-57" href="#v551-note-57">:link:</a></sup></sub> fix
