Correction to previous type change.

`timestamp`s do not work with `string`s at all, the data type must be `Date`

index.d.ts

@@ -316,7 +316,7 @@ declare namespace Eris {
     title?: string;
     description?: string;
     url?: string;
-    timestamp?: string | Date;
+    timestamp?: Date;
     color?: number;
     footer?: { text: string; icon_url?: string; proxy_icon_url?: string };
     image?: { url?: string; proxy_url?: string; height?: number; width?: number };