Fix 'method' keyword high-lighting

It makes more sense to add it to `keyword.function`, neovim does not
have a `keyword.method` category, and `method` is for the method name
itself.

test/highlight/object-oriented.tig

@@ -12,7 +12,7 @@ let
     var a := 12
 
     method meth() : int = self.a
-    /* <- keyword.method */
+    /* <- keyword.function */
     /*     ^ method */
     /*                    ^ variable.builtin */
   }