remove erroneous $

compare output of introspection
`&trait_mod:<is>.candidates».signature».raku.grep(/ \(\$\) /)».say`

Author: schultzdavid

doc/Type/Attribute.rakudoc

@@ -154,7 +154,7 @@ default accessor for the attribute will return a writable value.
 =head2 X<trait is readonly|Traits,is readonly>
 
     =for code :skip-test<listing>
-    multi trait_mod:<is> (Attribute:D $attr, :$readonly($)!)
+    multi trait_mod:<is> (Attribute:D $attr, :readonly($)!)
 
 Marks as C<readonly> an attribute of a class that has the C<is rw> trait.