Fix invalid Raku version in META6.json

Author: JRaspass

META6.json

@@ -6,7 +6,7 @@
   "description": "Raku core CSS grammars",
   "license": "Artistic-2.0",
   "name": "CSS::Grammar",
-  "perl": "6c",
+  "perl": "6.c",
   "provides": {
     "CSS::Grammar": "lib/CSS/Grammar.rakumod",
     "CSS::Grammar::AST": "lib/CSS/Grammar/AST.rakumod",