[mdspan.layout.left.cons] extents() should be other.extents() (cplusplus#6069)

The precondition was erroneously referring to the not yet
constructed extents instead of other.extents().

Note that the extents of the to be constructed layout will be
initialized with other.extents - i.e. after construction they will
return the same value for the fwd-prod-of-extents etc.

Author: crtrott

source/containers.tex

@@ -19442,7 +19442,7 @@
 If \tcode{extents_type::rank() > 0} is \tcode{true},
 then for all $r$ in the range $[0, \tcode{extents_type::rank()})$,
 \tcode{other.stride($r$)} equals
-\tcode{extents().\exposid{fwd-prod-of-extents}($r$)}, and
+\tcode{other.extents().\exposid{fwd-prod-of-extents}($r$)}, and
 \item
 \tcode{other.required_span_size()} is representable as
 a value of type \tcode{index_type}\iref{basic.fundamental}.
@@ -19700,7 +19700,7 @@
 If \tcode{extents_type::rank() > 0} is \tcode{true},
 then for all $r$ in the range $[0, \tcode{extents_type::rank()})$,
 \tcode{other.stride($r$)} equals
-\tcode{extents().\exposid{rev-prod-of-extents}($r$)}.
+\tcode{other.extents().\exposid{rev-prod-of-extents}($r$)}.
 \item
 \tcode{other.required_span_size()} is representable as
 a value of type \tcode{index_type}\iref{basic.fundamental}.