[meta.reflection.queries] Fix "r" that should be "T".

This is a follow-up to 2567873,
which fixed a missing paragraph, but forgot to make this change.

Author: tkoeppe

source/meta.tex

@@ -4245,7 +4245,7 @@
 
 \pnum
 \returns
-\tcode{true} if \tcode{r} represents a const or volatile type, respectively,
+\tcode{true} if \tcode{T} represents a const or volatile type, respectively,
 or a const- or volatile-qualified function type, respectively.
 Otherwise, \tcode{false}.
 \end{itemdescr}