[mdspan.sub.sub] Fix typo in submdspan Effect:.

The class `mdspan` doesn't have a member `data`, it has `data_handle`
(which returns a generalized pointer). This "data handle" is what's
passed, together with an offset, to the accessor to "shift the pointer".

The `AccessorPolify::offset_policy` is a type alias and therefore needs
a preceeding `typename`.

Co-authored-by: Jonathan Wakely <[email protected]>

Author: 1uc

source/containers.tex

@@ -26118,9 +26118,9 @@
 Equivalent to:
 \begin{codeblock}
 auto sub_map_result = submdspan_mapping(src.mapping(), slices...);
-return mdspan(src.accessor().offset(src.data(), sub_map_result.offset),
+return mdspan(src.accessor().offset(src.data_handle(), sub_map_result.offset),
               sub_map_result.mapping,
-              AccessorPolicy::offset_policy(src.accessor()));
+              typename AccessorPolicy::offset_policy(src.accessor()));
 \end{codeblock}
 \end{itemdescr}