[mdspan.layout.left.cons] Fix typo in precondition.

Here `other` is a layout mapping and layout mappings don't have this API
to access the `i`th extent. They only have `extents()` to get the
std::extents object.

Author: 1uc

source/containers.tex

@@ -22102,7 +22102,7 @@
 \begin{itemize}
 \item
 If \tcode{extents_type::rank() > 1} is \tcode{true},
-then \tcode{other.stride(1)} equals \tcode{other.extents(0)}.
+then \tcode{other.stride(1)} equals \tcode{other.extents().ex\-tent(0)}.
 \item
 \tcode{other.required_span_size()} is representable as
 a value of type \tcode{index_type}.