Minor wording change in security consideration

[Acconut]

Based on feedback from Mohamed Boucadair:

Section 4 includes a loose reco for operators, I would s/could/are recommended (or “are encouraged”).

[LPardue]

This is one I actually disagree with. It will raise questions about whether it needs to be RECOMMENDED and can be misread as the document recommending or encouraging operators to prefer a generic type, in which case why bother with this document at all.

I would prefer we leave the text as is

[Acconut]

That's a valid counterargument. Let's leave it as-is then.