[spanbuf.virtuals] Add missing "override" (#4795)

... to the declaration of `setbuf` to agree with the class synopsis.

Author: CaseyCarter

source/iostreams.tex

@@ -9665,7 +9665,7 @@
 
 \indexlibrarymember{setbuf}{basic_spanbuf}%
 \begin{itemdecl}
-basic_streambuf<charT, traits>* setbuf(charT* s, streamsize n);
+basic_streambuf<charT, traits>* setbuf(charT* s, streamsize n) override;
 \end{itemdecl}
 
 \begin{itemdescr}