remove "et al" from single- & two-author citations

from richard.droste@eng.ox.ac.uk
> - It should be "Chollet, 2016" instead of "Chollet et al., 2016" for the Xception model. François Chollet is the only author (see https://arxiv.org/abs/1610.02357).
> - It should be "Simonyan and Zisserman, 2014" instead of "Simonyan et al., 2014" for the VGG models (see https://arxiv.org/abs/1409.1556).