Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]

The URL

http://www.h-online.com/security/Symantec-reports-first-active-attack-on-a-DSL-router--/news/102352

points to an article titled 'The H is closing down'. The good one is:

http://www.h-online.com/security/news/item/Symantec-reports-first-active-attack-on-a-DSL-router-735883.html
Merge pull request #17716 from javiervidal/fixing_documentation

Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]