N3545 Editorial fix

Add const to the added constexpr function, as constexpr no longer
implies const, but did at the time the paper was proposed.

Author: Stefanus Du Toit

source/utilities.tex

@@ -10111,7 +10111,7 @@
     typedef T value_type;
     typedef integral_constant<T,v> type;
     constexpr operator value_type() const { return value; }
-    constexpr value_type operator()() { return value; }
+    constexpr value_type operator()() const { return value; }
   };
   typedef integral_constant<bool, true> true_type;
   typedef integral_constant<bool, false> false_type;