Remove mention of OIDC certification from Dex

Dex [isn't][1] [yet][2] a certified OpenID Provider implementation. As of
today, it's not on the [list maintained by the OpenID Foundation][3].

[1]: dexidp/dex#42
[2]: dexidp/dex#262
[3]: https://openid.net/certification/

Signed-off-by: Etienne Dysli Metref <etienne.dysli@gmail.com>

docs/openid.md

@@ -83,7 +83,7 @@ oidc_providers:
 
 ### Dex
 
-[Dex][dex-idp] is a simple, open-source, certified OpenID Connect Provider.
+[Dex][dex-idp] is a simple, open-source OpenID Connect Provider.
 Although it is designed to help building a full-blown provider with an
 external database, it can be configured with static passwords in a config file.