[expr.ass] Improve preposition

Author: opensdh

source/expressions.tex

@@ -6709,7 +6709,7 @@
 that \tcode{E1} is evaluated only once.
 Such expressions are deprecated
 if \tcode{E1} has \tcode{volatile}-qualified type; see~\ref{depr.volatile.type}.
-In \tcode{+=} and \tcode{-=},
+For \tcode{+=} and \tcode{-=},
 \tcode{E1} shall either have arithmetic type or be a pointer to a
 possibly cv-qualified completely-defined object type. In all other
 cases, \tcode{E1} shall have arithmetic type.