[mdspan.layout.left.overview] Reorder "explicit" and "constexpr"

The standard ordering is "constexpr explicit", not the other way
round. The paper P0009R18 contains a non-standard style, and other
instances had already been fixed. Only this one seems to have been
missed previously.

Author: tkoeppe

source/containers.tex

@@ -19043,8 +19043,8 @@
       constexpr explicit(@\seebelow@)
         mapping(const layout_right::mapping<OtherExtents>&) noexcept;
     template<class OtherExtents>
-      explicit(extents_type::rank() > 0)
-      constexpr mapping(const layout_stride::mapping<OtherExtents>&);
+      constexpr explicit(extents_type::rank() > 0)
+        mapping(const layout_stride::mapping<OtherExtents>&);
 
     constexpr mapping& operator=(const mapping&) noexcept = default;