[class] Fix unclear phrase "only one" to clearer phrase "at most one"

used in [class.union], to make it more obvious that a union can have
no active members.

Author: zygoloid

source/classes.tex

@@ -117,7 +117,7 @@
 A \term{union} is a class defined with the \grammarterm{class-key}
 \tcode{union};
 \indextext{access control!\idxcode{union} default member}%
-it holds only one data member at a time~(\ref{class.union}).
+it holds at most one data member at a time~(\ref{class.union}).
 \enternote
 Aggregates of class type are described in~\ref{dcl.init.aggr}.
 \exitnote