[ios.syn] Fix cross-reference to [fpos] (#7195)

Author: Quuxplusone

source/iostreams.tex

@@ -686,7 +686,7 @@
   // \ref{stream.types}, types
   using streamoff  = @\impdef@;
   using streamsize = @\impdef@;
-  // \ref{ios}, class template \tcode{fpos}
+  // \ref{fpos}, class template \tcode{fpos}
   template<class stateT> class fpos;
 
   // \ref{ios.base}, class \tcode{ios_base}