uid-safe: Set Promise type to string.

types/uid-safe/index.d.ts

@@ -8,7 +8,7 @@ declare namespace UID {
     export interface Generate {
 
         (byteLength: number, callback: (err: any, str: string) => any): void;
-        (byteLength: number): Promise<any>;
+        (byteLength: number): Promise<string>;
 
         sync(byteLength: number): string;
     }