q20::iter_reference_t: fix typo in comment

It's std::iter_reference_t we're alike to, not q20::iter_reference_t,
which we're conditionally defining here.

Amends e86e275.

Pick-to: 6.8
Change-Id: I4e8b7a0da8c5563564bcad85dc16b8fe203e2171
Reviewed-by: Giuseppe D'Angelo <[email protected]>

src/corelib/global/q20iterator.h

@@ -39,7 +39,7 @@ namespace q20 {
 #endif
 } // namespace q20
 
-// like q20::iter_reference_t
+// like std::iter_reference_t
 namespace q20 {
 #ifdef __cpp_lib_ranges
     using std::iter_reference_t;