[associative.reqmts.general] Replace undefined "multiple keys" with "equivalent keys" (#7874)

Author: xmcgcg

source/containers.tex

@@ -2738,7 +2738,7 @@
 when \tcode{X} supports unique keys,
 \item
 \tcode{a_eq} denotes a value of type \tcode{X}
-when \tcode{X} supports multiple keys,
+when \tcode{X} supports equivalent keys,
 \item
 \tcode{a_tran} denotes a value of type \tcode{X} or \tcode{const X}
 when the \grammarterm{qualified-id}