Remove some incorrect statements from the Grammars article

The dynamic variables section stated that grammars behave like classes,
but they don't; they have a completely different metamodel.

doc/Language/grammars.pod6

@@ -382,14 +382,11 @@ say GrammarAdvice.subparse("use regexes for significant whitespace by default")
 =head2 Attributes in grammars
 
 Attributes may be defined in grammars. However, they can only be accessed by
-methods; in effect, grammars are classes, so these attributes behave in much the
-same way as object attributes. Attempting to use them from within a token will
-throw an exception because tokens are methods of L<Match|/type/Match>, not of
-the grammar itself; C<Match>es, in effect, behave as instances of the class that
-is the grammar. Note that mutating an attribute from within a method called in a
-token will I<only modify the attribute for that token's own match object>!
-Grammar attributes can be accessed in the match returned after parsing if made
-public:
+methods. Attempting to use them from within a token will throw an exception
+because tokens are methods of L<Match|/type/Match>, not of the grammar itself.
+Note that mutating an attribute from within a method called in a token will
+I<only modify the attribute for that token's own match object>! Grammar
+attributes can be accessed in the match returned after parsing if made public:
 
 =begin code
 grammar HTTPRequest {