[concepts.callable.general] Replace "function objects" with "callable types"

[eisenwave]

Fixes #8241.

The introductory paragraph is misleading: we claim that the subsequent concepts are for function objects, but the very next concept is invocable, which also accepts member function pointers and data member pointers, which are callable types but not function objects.

This seems like an editorial issue, since it's just about some intro fluff, nothing of normative consequence.

[morinmorin]

Would "callable types, function types, and references to such types" be more accurate?

[eisenwave]

I guess yeah. I'm pretty sure it's possible to put function references into std::invocable, but those are not defined to be "function objects".

That's a problem that also affects the contents of the subclause though, not just the intro paragraph.

[morinmorin]

Got it, thanks for the comment! I'll create a small follow-up PR soon.