Chore fix to apply best practices

[kenfdev]

Some simple fixes to apply the best practices. I was a bit skeptic about the version written in x.y but looking at the slack orb, it actually means to write as x.y. Hence I changed it.

https://github.com/CircleCI-Public/slack-orb/blob/staging/src/examples/approval.yml#L7

Also added a description of the access token.

Related issue: #8

[kenfdev]

I'll squash the commits if appropriate ;)

[pgavlin]

cc @chrsmith