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

Author: eisenwave

source/concepts.tex

@@ -1222,8 +1222,8 @@
 \rSec2[concepts.callable.general]{General}
 
 \pnum
-The concepts in \ref{concepts.callable} describe the requirements on function
-objects\iref{function.objects} and their arguments.
+The concepts in \ref{concepts.callable} describe the requirements on
+callable types\iref{func.def} and their arguments.
 
 \rSec2[concept.invocable]{Concept \cname{invocable}}