replace mail options reference URL at configuration.yml.example

wiki.rubyonrails.org seems dead.

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013037
Merge pull request #1083 from marutosi/finn-mail-conf

replace mail options reference URL at configuration.yml.example