Correct link in frozen pages' papers

There are two links missing:

./frozen/synapse.md at line 10 https://git-r3lab.uni.lu/core-services/r3-pages/blob/master/frozen/synapse.md
./frozen/machine_learning_assisted_neurotoxicity.md at line 10 https://git-r3lab.uni.lu/core-services/r3-pages/blob/master/frozen/machine_learning_assisted_neurotoxicity.md