LWG3431 <=> for containers should require three_way_comparable<T> instead of <=>

Author: burblebee

source/containers.tex

@@ -887,7 +887,7 @@
 
 \pnum
 \expects
-Either \tcode{<=>} is defined for values of type (possibly const) \tcode{T},
+Either \tcode{T} models \libconcept{three_way_comparable},
 or \tcode{<} is defined for values of type (possibly const) \tcode{T} and
 \tcode{<} is a total ordering relationship.