[streambuf.cons] Fix grammar typo

Author: jensmaurer

source/iostreams.tex

@@ -3108,7 +3108,7 @@
 \item
 the
 \tcode{getloc()}
-member to a copy the global locale,
+member to a copy of the global locale,
 \tcode{locale()},
 at the time of construction.
 \end{itemize}