snippetgen: add 2+ placeholder values for list field values

[busunkim96]

Adding more than one value makes it more visually clear the field is expected to be a list.

From discussion in #941 (comment)

    # Initialize request argument(s)
    request = credentials_v1.GenerateAccessTokenRequest(
        name="projects/{project}/serviceAccounts/{service_account}",
        scope=['scope_value_1', 'scope_value_2'],