Remove an overfull hbox.

doc/main.tex

@@ -2236,7 +2236,7 @@ type 'a mylist = 'a list =
   [@@deriving visitors { variety = "map" }]
 \end{lstlisting}
 \end{origenv}
-
+%
 If desired, instead of \oc|[@name]|, one can use
 \oc|[@visitors.name]| or \oc|[@deriving.visitors.name]|.