Update noise3Dgrad.glsl

Changed modification date in header (long overdue)

Author: stegu

src/noise3Dgrad.glsl

@@ -3,7 +3,7 @@
 //               noise functions.
 //      Author : Ian McEwan, Ashima Arts.
 //  Maintainer : stegu
-//     Lastmod : 20110822 (ijm)
+//     Lastmod : 20201014 (stegu)
 //     License : Copyright (C) 2011 Ashima Arts. All rights reserved.
 //               Distributed under the MIT License. See LICENSE file.
 //               https://github.com/ashima/webgl-noise