Add another "igonore that 403" entry

It regularly fails but is still valid.

mkdocs.yml

@@ -45,7 +45,7 @@ plugins:
       raise_error_excludes:
         # These regularly give us 403 when we try to check, but they are fine.
         # At least at the time we are writing this comment.
-        403: ["https://www.reddit.com/*", "https://vimeo.com/*", "https://www.raspberrypi.org/*", "https://linux.die.net/*", "https://ponylang101.splashthat.com/*", "https://www.researchgate.net/publication/221496370_Traits_Composable_Units_of_Behaviour" ]
+        403: ["https://www.reddit.com/*", "https://vimeo.com/*", "https://www.raspberrypi.org/*", "https://linux.die.net/*", "https://ponylang101.splashthat.com/*", "https://www.researchgate.net/publication/221496370_Traits_Composable_Units_of_Behaviour", "https://www.gophercon-russia.ru/en" ]
         429: ["https://github.com/*", "https://medium.com/*", "https://*.medium.com/*" ]
         404: ["https://github.com/organizations/ponylang/settings/repository-defaults", "https://twitter.com/bpdp/status/905653663913218050", "https://twitter.com/matthias_wahl/status/1002638819177435138"]