[move.iter.op.index] Fix misnested \tcode

Fixes #1459.

Author: tkoeppe

source/iterators.tex

@@ -2426,7 +2426,7 @@
 
 \begin{itemdescr}
 \pnum
-\returns \tcode{std::move(}current[n]).
+\returns \tcode{std::move(current[n])}.
 \end{itemdescr}
 
 \rSec4[move.iter.op.comp]{\tcode{move_iterator} comparisons}