* src/bison.simple (yydebug): Fix the comment.

From Bruce Lilly.

src/bison.simple

@@ -209,10 +209,8 @@ do {						\
   if (yydebug)					\
     YYFPRINTF Args;				\
 } while (0)
-/* Nonzero means print parse trace. [The following comment makes no
-   sense to me.  Could someone clarify it?  --akim] Since this is
-   uninitialized, it does not stop multiple parsers from coexisting.
-   */
+/* Nonzero means print parse trace.  It is left uninitialized so that
+   multiple parsers can coexist.  */
 int yydebug;
 #else /* !YYDEBUG */
 # define YYDPRINTF(Args)