Fix typo in reference.md

Author: kulakowski

src/doc/reference.md

@@ -522,7 +522,7 @@ The two values of the boolean type are written `true` and `false`.
 ### Symbols
 
 ```{.ebnf .gram}
-symbol : "::" "->"
+symbol : "::" | "->"
        | '#' | '[' | ']' | '(' | ')' | '{' | '}'
        | ',' | ';' ;
 ```