[range.refinements] Fix template argument name ("T", not "R")

Author: tkoeppe

source/ranges.tex

@@ -1624,7 +1624,7 @@
 \begin{itemdecl}
 template<class T>
   concept @\deflibconcept{constant_range}@ =
-    @\libconcept{input_range}@<T> && @\exposconcept{constant-iterator}@<iterator_t<R>>;
+    @\libconcept{input_range}@<T> && @\exposconcept{constant-iterator}@<iterator_t<T>>;
 \end{itemdecl}
 
 \rSec1[range.utility]{Range utilities}