Fix symbol in grammar; this should fix some hyperlinking in the HTML

version.

Doc/ref/ref6.tex

@@ -62,7 +62,7 @@ Assert statements\stindex{assert} are a convenient way to insert
 debugging assertions\indexii{debugging}{assertions} into a program:
 
 \begin{productionlist}
-  \production{assert_statement}
+  \production{assert_stmt}
              {"assert" \token{expression} ["," \token{expression}]}
 \end{productionlist}