Update sample Ensure the Site Assets Library is created.

The sample does not work in the latest versions of the CLI for Microsoft 365, as the default output has switched to json, some time back. Retrieving an access token retrieves a string with double quotes in it, which cannot be used as a bearer token as such.

The sample could also be shortened by refactoring retrieving the list to a one-liner. Retrieving a list by title has been possible since some time.