[common.iter.cust] Pluralize subclause heading (#3995)

to match [counted.iter.cust]

Author: NinaRanns

source/iterators.tex

@@ -5044,7 +5044,7 @@
 $i$ is \tcode{x.v_.index()} and $j$ is \tcode{y.v_.index()}.
 \end{itemdescr}
 
-\rSec3[common.iter.cust]{Customization}
+\rSec3[common.iter.cust]{Customizations}
 
 \indexlibrarymember{iter_move}{common_iterator}%
 \begin{itemdecl}