Update GeneratePresignedURL.java

Updating the code sample not to use java.util.Date since it is good coding practice not to use that.

Author: DBirtolo-AMZ

java/example_code/s3/src/main/java/aws/example/s3/GeneratePresignedURL.java

@@ -34,6 +34,7 @@
 
 import java.io.IOException;
 import java.net.URL;
+import java.time.Instant;
 
 public class GeneratePresignedURL {
 
@@ -49,8 +50,7 @@ public static void main(String[] args) throws IOException {
                     .build();
 
             // Set the presigned URL to expire after one hour.
-            java.util.Date expiration = new java.util.Date();
-            long expTimeMillis = expiration.getTime();
+            long expTimeMillis = Instant.now().toEpochMilli();
             expTimeMillis += 1000 * 60 * 60;
             expiration.setTime(expTimeMillis);
 
@@ -75,4 +75,4 @@ public static void main(String[] args) throws IOException {
     }
 }
 
-// snippet-end:[s3.java.generate_presigned_url.complete]
+// snippet-end:[s3.java.generate_presigned_url.complete]