[linalg.transp.layout.transpose] Fix misplaced data members of layout_transpose::mapping (#8423)

The intent is that _`nested-mapping_`_ and _`extents_`_ belong to
`layout_transpose::mapping` but not `layout_transpose`. This was a
mistake in the original paper P1673R13, confirmed by the author, and
it can also be inferred from their usages.

Author: frederick-vs-ja

source/numerics.tex

@@ -13040,11 +13040,11 @@
 
       template<class OtherExtents>
         friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y);
-    };
 
-  private:
-    @\exposid{nested-mapping-type}@ @\exposid{nested-mapping_}@;                         // \expos
-    extents_type @\exposid{extents_}@;                                       // \expos
+    private:
+      @\exposid{nested-mapping-type}@ @\exposid{nested-mapping_}@;                       // \expos
+      extents_type @\exposid{extents_}@;                                     // \expos
+    };
   };
 }
 \end{codeblock}