P2833R2-Part2 Freestanding Library: inout expected span

This applies P2833R2's change to [views.span] after merging P2821R5.

Author: burblebee

source/containers.tex

@@ -18321,7 +18321,7 @@
 
     // \ref{span.elem}, element access
     constexpr reference operator[](size_type idx) const;
-    constexpr reference at(size_type idx) const;
+    constexpr reference at(size_type idx) const;                            // freestanding-deleted
     constexpr reference front() const;
     constexpr reference back() const;
     constexpr pointer data() const noexcept;