Merge #1410

1410: Updated url for cert-manager doc r=mergify[bot] a=tuxfanou

Provided url does not exists anymore.

Replace pull request #1404 

Co-authored-by: Stéphane Burdin <steph@tux.tf>

docs/kubernetes/mailu/index.rst

@@ -191,7 +191,7 @@ Happy mailing!
 
 .. _here: https://github.com/hacor/Mailu/blob/master/core/postfix/conf/main.cf#L35
 .. _cert-manager: https://github.com/jetstack/cert-manager
-.. _docs: https://cert-manager.readthedocs.io/en/latest/getting-started/2-installing.html
+.. _docs: https://cert-manager.io/docs/installation/kubernetes/#installing-with-helm
 
 Imap login fix
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