update land productivity class definition to match UNCCD's

[gdaldegan]

Need to update the Productivity class definition on the Load a Custom Land Productivity Dataset to match UNCCD's class terminology:

• Declining = 1
• Moderate decline = 2
• Stressed = 3
• Stable = 4
• Increasing = 5
• No data = 0 or -3276

[azvoleff]

Thanks - @gdaldegan fixed in 25e65f676af7c32f08ed060d7055b92c75f00402