[range.elements.view] fix class-key for iterator and sentinel

Author: jwakely

source/ranges.tex

@@ -8815,10 +8815,10 @@
 
   private:
     // \ref{range.elements.iterator}, class template \tcode{elements_view::\exposid{iterator}}
-    template<bool> struct @\exposid{iterator}@;                     // \expos
+    template<bool> class @\exposid{iterator}@;                      // \expos
 
     // \ref{range.elements.sentinel}, class template \tcode{elements_view::\exposid{sentinel}}
-    template<bool> struct @\exposid{sentinel}@;                     // \expos
+    template<bool> class @\exposid{sentinel}@;                      // \expos
 
     V @\exposid{base_}@ = V();                                      // \expos
   };