[span.iterators] Fix cross-reference to container iterators (#6183)

The current cross-reference is to [containers.requirements], which
is the whole containers requirements clause, including not just
general containers, but also allocator-aware, reversible, sequence,
associative, and unodered associative containers.  It seems very
unlikely that the cross-reference expects to be the intersection
of all of those.

Rather, the reference seems to intend just the [containers.reqmts]
subclause, which adds just two useful constraints: random access
iterators should support the 3-way comparison operator, and the
interoperabiity of container iterators and const_iterators.

Author: AlisdairM

source/containers.tex

@@ -18382,7 +18382,7 @@
 whose reference type is \tcode{reference}.
 
 \pnum
-All requirements on container iterators\iref{container.requirements} apply to
+All requirements on container iterators\iref{container.reqmts} apply to
 \tcode{span::iterator} as well.
 \end{itemdescr}