[spanstream.ctor] Fix base class name in effects

Author: jwakely

source/iostreams.tex

@@ -10073,7 +10073,7 @@
 \pnum
 \effects
 Initializes the base class with
-\tcode{basic_ostream<charT, traits>(addressof(sb))}
+\tcode{basic_iostream<charT, traits>(addressof(sb))}
 and \tcode{sb} with
 \tcode{basic_spanbuf<charT, traits>(s, which)}\iref{spanbuf.ctor}.
 \end{itemdescr}