Fix docstring formatting on SlackHook

[jlaneve]

Fixes formatting on SlackHook's docstring to adhere to Sphinx standards. See the Airflow docs for the current rendering - the short description is currently:

"Creates a Slack connection, to be used for calls. Takes both Slack API token directly and connection that has Slack API token. If both supplied, Slack API token will be used. Exposes also the rest of slack.WebClient args Examples:"

Also fixes minor grammatical issues.

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest master or amend the last commit of the PR, and push it with --force-with-lease.

[potiuk]

You need reformat the files - staic checks are failing.

[drdenzy]

If you are using VS Code, you can use black to reformat the file and that should fix the static check error.

[potiuk]

Or even better. Run pre-commit install and it will be automatically fixed for you when you run git commit

[jlaneve]

@potiuk I think they're fixed now but there's one task still failing. I can't tell what the error is / if it's caused by my fix. It doesn't make sense that it would be given it's a Postgres failure but can you confirm?

[kaxil]

Thanks @jlaneve for the PR