[range.utility.helper] Add missing '='

Author: tkoeppe

source/ranges.tex

@@ -1363,7 +1363,7 @@
     @\libconcept{input_iterator}@<I> && (is_pointer_v<I> || requires(I i) { i.operator->(); });
 
 template<class T, class U>
-  concept @\defexposconcept{different-from}@                        // \expos
+  concept @\defexposconcept{different-from}@ =                      // \expos
     !@\libconcept{same_as}@<remove_cvref_t<T>, remove_cvref_t<U>>;
 \end{codeblock}