BUG: fix the relaxivity value

The updated value is based on the following reference:

Pintaske J, Martirosian P, Graf H, Erb G, Lodemann K-P, Claussen CD, Schick F.
Relaxivity of Gadopentetate Dimeglumine (Magnevist), Gadobutrol (Gadovist), and
Gadobenate Dimeglumine (MultiHance) in human blood plasma at 0.2, 1.5, and 3
Tesla. Investigative radiology. 2006 March;41(3):213–21.
Merge pull request #7 from fedorov/fix-r1-relaxivity

BUG: fix the relaxivity value