Fix typo in submdspan_extents.

The pack `Extents` must be a pack of `size_t` integers.

Author: 1uc

source/containers.tex

@@ -25500,15 +25500,15 @@
 
 \indexlibraryglobal{submdspan_extents}%
 \begin{itemdecl}
-template<class IndexType, class... Extents, class... SliceSpecifiers>
+template<class IndexType, size_t... Extents, class... SliceSpecifiers>
   constexpr auto submdspan_extents(const extents<IndexType, Extents...>& src,
                                    SliceSpecifiers... slices);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
 \constraints
-\tcode{sizeof...(slices)} equals \tcode{Extents::rank()}.
+\tcode{sizeof...(slices)} equals \tcode{sizeof...(Extents)}.
 
 \pnum
 \mandates