[fpos.operations] Fix name of type trait

Author: jensmaurer

source/iostreams.tex

@@ -1681,7 +1681,7 @@
 \oldconcept{Destructible} (\tref{cpp17.destructible}) requirements.
 If \tcode{is_trivially_copy_constructible_v<stateT>} is \tcode{true},
 then \tcode{fpos<stateT>} has a trivial copy constructor.
-If \tcode{is_trivially_copy_assignable<stateT>} is \tcode{true},
+If \tcode{is_trivially_copy_assignable_v<stateT>} is \tcode{true},
 then \tcode{fpos<stateT>} has a trivial copy assignment operator.
 If \tcode{is_trivially_destructible_v<stateT>} is \tcode{true},
 then \tcode{fpos<stateT>} has a trivial destructor.