Update Golang documentation to use latest stable version (1.14)

[jbleduigou]

This is very similar to my previous PR:
#1657

I am wondering if we could automate that somehow?

[tianon]

Yeah, I guess it would be interesting to create some kind of either %%LATEST%% template variable that references the "most specific" tag of the foo:latest group or maybe some way to specify a bashbrew cat --format string inside the templates (which would be much more flexible). I bet there are a bunch of images where such a thing would be useful, although it's a bit complicated to implement.

However, thanks for the contributions in the meantime! 👍