[mdspan.layout.stride.cons] Convert to integral type before comparing to 0

`s[i]` is an lvalue of a type that we can only convert to `index_type`; clearly the wording intends that the result of the conversion should be `> 0`.

Author: CaseyCarter

source/containers.tex

@@ -20513,7 +20513,8 @@
 \expects
 \begin{itemize}
 \item
-\tcode{s[$i$] > 0} is \tcode{true}
+The result of converting \tcode{s[$i$]} to \tcode{index_type}
+is greater than \tcode{0}
 for all $i$ in the range $[0, \exposid{rank_})$.
 \item
 \tcode{\exposid{REQUIRED-SPAN-SIZE}(e, s)} is representable