[variant.variant.general] Fix typo in "constexpr" (#4765)

Author: hewillk

source/utilities.tex

@@ -4065,7 +4065,7 @@
       constexpr explicit variant(in_place_index_t<I>, initializer_list<U>, Args&&...);
 
     // \ref{variant.dtor}, destructor
-    constepxr ~variant();
+    constexpr ~variant();
 
     // \ref{variant.assign}, assignment
     constexpr variant& operator=(const variant&);