clarified comment for author.stackoverflow value in _config.yml

[rschaerer]

Initial Situation

• The author.stackoverflow variable was commentet with # https://stackoverflow.com/users/123456/username, which led me to the conclusion that I should add the whole url as value.
• The stackoverflow combination of numeric id / username isn't self-explanatory.

Proposed Change

Change the comment to explicit state the requested value "123456/username" and append the whole demo url to show where the needed value can be found.

[mmistakes]

Thanks. Merged into develop and will push with the next theme release.

ref: d3ac112