Fix `:knative_eventing_doc_url:` extra '/' and broken link

Summary
The `:knative_eventing_doc_url:` was defined with slash '/' at the end of the URL, but later in the text, it was used with extra '/', which resulted in double slash '//', which could sometimes break link.
Another problem was with wrong link to "Knative triggers".

Acceptance criteriea
All links are working correctly.