[fpos] Fix typo in exposition-only member

Author: jensmaurer

source/iostreams.tex

@@ -1636,7 +1636,7 @@
     // \ref{fpos.members}, members
     stateT state() const;
     void state(stateT);
-  private;
+  private:
     stateT st;                  // \expos
   };
 }