[tuple.syn] Fix return type of ignore-type::operator= (#7840)

This fixes a misapplication of P2968R2, in commit 225eadc.

Author: Tsche

source/utilities.tex

@@ -1549,7 +1549,7 @@
 
   // \tcode{ignore}
   struct @\exposidnc{ignore-type}@ {                      // \expos
-    constexpr const @\exposid{ignore-type}@
+    constexpr const @\exposid{ignore-type}@&
       operator=(const auto &) const noexcept { return *this; }
   };
   inline constexpr @\exposid{ignore-type}@ ignore;