Override data description Climate Chapter 1

[olafveerman]

Until multiple legends are supported, we should update the description to read something like:

Left: The five year baseline shows changes in nitrogen dioxide levels. Redder colors indicate increases in NO₂. Bluer colors indicate lower levels of NO₂. Right: This layer shows changes in nitrogen dioxide (NO₂) levels in µmol/m₂. Redder colors indicate increases in NO₂. Bluer colors indicate lower levels of NO₂. Missing pixels indicate areas of no data most likely associated with cloud cover or snow.