Fix misleading usage examples for config:set and config:delete commands

[BaluErtl]

I fell into that mistake that naively believed the syntax the usage hint suggested and ran this command like this:
$ drush config:set system.site page.403 filter/tips
When tested the results in browser, I got the regular exception from Drupal core:

The website encountered an unexpected error. Please try again later.
InvalidArgumentException: The user-entered string 'filter/tips' must begin with a '/', '?', or '#'. in Drupal\Core\Url::fromUserInput() (line 204 of core/lib/Drupal/Core/Url.php).

Although Drupal's GUI still lacks of consistency on how to properly request users to type in internal URL paths (with or without leading slashes), at least on the CLI side Drush could do better. Let's help future fellows not to fall again into this pitfall I learned today. Once they took the time to check --help before running the actual command, that examples should really help them, not misleading.