[mdspan.layout.right.obs] Add missing constexpr to required_span_size (cplusplus#7872)

Author: 1uc

source/containers.tex

@@ -22491,7 +22491,7 @@
 
 \indexlibrarymember{required_span_size}{layout_right::mapping}%
 \begin{itemdecl}
-index_type required_span_size() const noexcept;
+constexpr index_type required_span_size() const noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}