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 e86e2752845ede635840851198c334f550fafb8f.

Change-Id: I4e8b7a0da8c5563564bcad85dc16b8fe203e2171
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from commit c3de0905697f8709f208c5cc1100b4f5e45032f1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

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;