[expr.add] Reword specification for std::ptrdiff_t

Author: frederick-vs-ja

source/expressions.tex

@@ -6273,7 +6273,7 @@
 \indextext{comparison!undefined pointer}%
 When two pointer expressions \tcode{P} and \tcode{Q} are subtracted,
 the type of the result is an \impldef{type of \tcode{ptrdiff_t}} signed
-integral type; this type shall be the same type that is defined as
+integral type; this type shall be the same type that is named by
 \tcode{std::ptrdiff_t} in the \libheader{cstddef}
 header\iref{support.types.layout}.
 \begin{itemize}