[alg.partitions] Replace non-existent concept "copy_assignable" concept with "copyable"

Author: eisenwave

source/algorithms.tex

@@ -8005,7 +8005,7 @@
 \oldconcept{CopyConstructible} and \oldconcept{CopyAssignable} requirements.
 For the parallel algorithm overloads in namespace \tcode{ranges},
 there can be a performance cost if \tcode{iter_value_t<I>}
-does not model both \libconcept{copy_constructible} and \tcode{copy_assignable}.
+does not model \libconcept{copyable}.
 \end{note}
 
 \pnum