Fix proto link broken.

Close issue #1899

Author: fluca1978

doc/Language/grammars.pod6

@@ -96,7 +96,7 @@ conjunction with the grammar.
 
 L<Grammar|/type/Grammar>s are composed of rules, tokens and regexes;
 there are actually methods, since grammars are classes. These methods
-can have things in common, and thus can use L<proto>.
+can have things in common, and thus can use L<proto|/syntax/proto>.
 
 For instance, if you have a lot of alternations, it may become difficult to produce
 readable code or subclass your grammar. In the Actions class below, the