[intro.races] Harmonize index entry for "visible side effect(s)".

Author: tkoeppe

source/basic.tex

@@ -6022,7 +6022,7 @@
 \end{note}
 
 \pnum
-A \defnadj{visible}{side effect} $A$ on a scalar object or bit-field $M$
+A \defnadjx{visible}{side effect}{side effects} $A$ on a scalar object or bit-field $M$
 with respect to a value computation $B$ of $M$ satisfies the
 conditions:
 \begin{itemize}