Fix macOS Print Screen

Many years ago this value was changed from F13 to Print Screen with
1ae9ce1e0a.  However, this was later
reverted to F13 with 5759d0fdf2.

This changes it back to Print Screen for consistency with other
platforms.

Closes #1786.

src/cocoa_init.m

@@ -251,7 +251,7 @@ static void createKeyTables(void)
     _glfw.ns.keycodes[0x6D] = GLFW_KEY_F10;
     _glfw.ns.keycodes[0x67] = GLFW_KEY_F11;
     _glfw.ns.keycodes[0x6F] = GLFW_KEY_F12;
-    _glfw.ns.keycodes[0x69] = GLFW_KEY_F13;
+    _glfw.ns.keycodes[0x69] = GLFW_KEY_PRINT_SCREEN;
     _glfw.ns.keycodes[0x6B] = GLFW_KEY_F14;
     _glfw.ns.keycodes[0x71] = GLFW_KEY_F15;
     _glfw.ns.keycodes[0x6A] = GLFW_KEY_F16;