[mdspan.layout.stride.obs] Add missing parentheses

Author: tkoeppe

source/containers.tex

@@ -19852,7 +19852,7 @@
 \tcode{true} if \tcode{x.extents() == y.extents()} is \tcode{true},
 \tcode{\exposid{OFFSET}(y) == 0} is \tcode{true}, and
 each of \tcode{x.stride($r$) == y.stride($r$)} is \tcode{true}
-for $r$ in the range  $[0, \tcode{x.extents.rank()})$.
+for $r$ in the range  $[0, \tcode{x.extents().rank()})$.
 Otherwise, \tcode{false}.
 \end{itemdescr}