The jailor has to make two choices: which urn does he choose from, and which disk in that urn does he choose.

Lets say we leave the disks as they are. The total probability of choosing a green disk is:

1/2*(10/10) + 1/2*(0/10) = 1/2

Lets now take one green disk and put it in the other urn.

1/2*(9/9) + 1/2*(1/11) = 1/2 + 1/22 = 6/11

This improves our odds slightly. Lets continue putting green disks in the other jar until there is only one green disk left:

1/2*(1/1) + 1/2*(9/19) = 1/2 + 9/38 = 14/19

This is the best we can do; we now have a 73% chance of the jailor choosing a green disk.