Update year to 2020

LICENSE

@@ -1,6 +1,6 @@
 zlib License
 	
-Copyright (c) 2014-2019 Ramon Santamaria (@raysan5)
+Copyright (c) 2014-2020 Ramon Santamaria (@raysan5)
 
 This software is provided "as-is", without any express or implied warranty. In no event 
 will the authors be held liable for any damages arising from the use of this software.

src/raygui.h

@@ -106,7 +106,7 @@
 *
 *   LICENSE: zlib/libpng
 *
-*   Copyright (c) 2014-2019 Ramon Santamaria (@raysan5)
+*   Copyright (c) 2014-2020 Ramon Santamaria (@raysan5)
 *
 *   This software is provided "as-is", without any express or implied warranty. In no event
 *   will the authors be held liable for any damages arising from the use of this software.