The init command should use `docs` instead of `doc` as the default target

[aklin]

Is your feature request related to a problem? Please describe.
Using adrjs init will intialise a doc directory as per #15.
The default documents directory on github is docs.

Describe the solution you'd like
Change the default to docs.

Describe alternatives you've considered
Manually renaming the folder and the contents of .adr-dir.

Additional context
We cannot do this in v1 because of ADR-5, it breaks 1-1 compatibility with adrtools.