Unclear example values / comments on external services configuration editor

Created by: KattMingMing

• Sourcegraph version: 3.0-beta

Editing a config file to add my access token was alright, but a sample line of a correct token would make the expected format more clear.

Feedback from: https://twitter.com/pjargon/status/1089530124666470400

On the GitHub External Services page we currently show the following documentation:

{
  // Use Ctrl+Space for completion, and hover over JSON properties for documentation.
  // Configuration options are documented here:
  // https://docs.sourcegraph.com/admin/site_config/all#githubconnection-object

  "url": "https://github.com",

  // A token is required for access to private repos, but is also helpful for public repos
  // because it grants a higher hourly rate limit to Sourcegraph.
  "token": "<personal access token with repo scope (https://github.com/settings/tokens/new)>",

  // Sync public repositories from https://github.com by adding them to "repos".
  // (This is not necessary for GitHub Enterprise instances)
  // "repos": [
  //     "sourcegraph/sourcegraph"
  // ]

}```