typings(Util): use StringResolvable (fixes old pull) (discordjs#3556)

Fixes my extremely old pull discordjs#3212 that didn't actually update the typing (Didn't know at the time.)

typings/index.d.ts

@@ -1447,7 +1447,7 @@ declare module 'discord.js' {
 			route: object,
 			reason?: string
 		): Promise<{ id: Snowflake; position: number }[]>;
-		public static splitMessage(text: string, options?: SplitOptions): string[];
+		public static splitMessage(text: StringResolvable, options?: SplitOptions): string[];
 		public static str2ab(str: string): ArrayBuffer;
 	}