text: Title: Improve WebClient's slack.com url overwriting experience
  Body: (Filling out the following details about bugs will help us solve your issue sooner.)
  
  #### Steps to reproduce:
  
  (Share the commands to run, source code, and project settings (e.g., setup.py))
  
  1. Try setting up an app to point to a different slack url that does not include a trailing `/`
  2. `app = App(token=os.environ.get("SLACK_BOT_TOKEN"), client=WebClient(os.environ.get("SLACK_BOT_TOKEN"), "https://example.slack.com/api"))`
  3. This will fail 🔴 
  4. Add a trailing `/`
  `app = App(token=os.environ.get("SLACK_BOT_TOKEN"), client=WebClient(os.environ.get("SLACK_BOT_TOKEN"), "https://example.slack.com/api/"))`
  5. This will pass 🟢 
  
  ### Expected result:
  
  `app = App(token=os.environ.get("SLACK_BOT_TOKEN"), client=WebClient(os.environ.get("SLACK_BOT_TOKEN"), "https://example.slack.com/api"))`
  
  Should provide an informative error or format the slack URL to work
  
  ## Requirements
  
  Please read the [Contributing guidelines](https://github.com/slackapi/bolt-python/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.
  