Fix Claim of Carbon Savings on "TBC Medal" Site

There are a few claims on the TBC medal site that (I think) are incorrect. Please double check my math/chemistry, but there's a claim that 1 gallon of gas leads to "20 pounds of carbon" in the atmosphere. I think the correct claim is that 1 gallon of gas leads to "20 pounds of carbon DIOXIDE" in the atmosphere - which only contains about 6 pounds of carbon.

Could be totally wrong, only using high school chemistry here.