[span.streams.overview] Fix typo in "these" (#4769)

Author: CaseyCarter

source/iostreams.tex

@@ -9301,7 +9301,7 @@
 that associate stream buffers with objects whose types are
 specializations of \tcode{span} as described in \ref{views.span}.
 \begin{note}
-A user of theses classes is responsible for ensuring
+A user of these classes is responsible for ensuring
 that the character sequence represented by the given \tcode{span}
 outlives the use of the sequence by objects of the classes in subclause \ref{span.streams}.
 Using multiple \tcode{basic_spanbuf} objects