[sequence.reqmts] Use "program-defined sequence container" consistently.

The earlier wording of "containers that the user defines" was not
retained for new additions to the Standard, and it is needlessly
informal. The term "program-defined" was only added recently, whereas
the original wording has essentially existed since the beginnig.

Author: tkoeppe

source/containers.tex

@@ -1238,7 +1238,7 @@
 \tcode{array} is provided as a sequence container which provides limited sequence operations
 because it has a fixed number of elements. The library also provides container adaptors that
 make it easy to construct abstract data types, such as \tcode{stack}s, \tcode{queue}s, \tcode{flat_map}s, or \tcode{flat_multimap}s, out of
-the basic sequence container kinds (or out of other kinds of sequence containers that the user defines).
+the basic sequence container kinds (or out of other program-defined sequence containers).
 
 \pnum
 \begin{note}