[optional.swap] Fix singular->plural tense that was missed in LWG4015.

Author: burblebee

source/utilities.tex

@@ -4060,7 +4060,7 @@
 \pnum
 If any exception is thrown, the results of the expressions \tcode{this->has_value()} and \tcode{rhs.has_value()} remain unchanged.
 If an exception is thrown during the call to function \tcode{swap},
-the state of \exposid{val} and \tcode{rhs.\exposid{val}} is determined by the exception safety guarantee of \tcode{swap} for lvalues of \tcode{T}.
+the states of \exposid{val} and \tcode{rhs.\exposid{val}} are determined by the exception safety guarantee of \tcode{swap} for lvalues of \tcode{T}.
 If an exception is thrown during the call to \tcode{T}'s move constructor,
 the states of \exposid{val} and \tcode{rhs.\exposid{val}} are determined by the exception safety guarantee of \tcode{T}'s move constructor.
 \end{itemdescr}