fix: apply enum for ruling source (#1)

Author: crookedneighbor

src/objects/Ruling/Ruling.ts

@@ -14,7 +14,7 @@ export type ScryfallRuling = ScryfallObject.Object<ScryfallObject.ObjectType.Rul
   /**
    * A computer-readable string indicating which company produced this ruling, either `wotc` or `scryfall`
    */
-  source: string;
+  source: "wotc" | "scryfall";
   /**
    * The date when the ruling or note was published
    */