Fix link placeholder in FF rollout template

What does this MR do and why?

Looks like the [main-issue] reference was removed here, but some links were still referencing it. Noticed this in my first own FF rollout here, where I pasted the output from the bin/feature-flag command and had a few [the feature issue][main-issue] occurrences.

I've replaced these occurrences simply with the placeholder used in parts that got replaced by the command successfully.