Add missing paren to menu link plugin

[virajmparekh]

When you run astro airflow init, the example menu_link is initialized with a missing plugin:

virajparekh@orbiter:~/Code/Astronomer/Other/new_cli_test/plugins$ cat plugins/example-plugin.py 
from airflow.plugins_manager import AirflowPlugin
from flask_admin.base import MenuLink

"""
Look for the Astronomer tab in the UI.
"""
airflow_plugins_ml = MenuLink(
    category='Astronomer',
    name='Airflow-Plugins',
    url='https://github.com/airflow-plugins/')

astro_docs_ml = MenuLink(
    category='Astronomer',
    name='Astronomer Docs',
    url='https://www.astronomer.io/docs/')
    
astro_guides_ml = MenuLink(
    category='Astronomer',
    name='Airflow Guides',
    url='https://www.astronomer.io/guides/'

class AstroLinksPlugin(AirflowPlugin):
    name = 'astronomer_menu_links'
    operators = []
    flask_blueprints = []
    hooks = []
    executors = []
    macros = []
    admin_views = []
    menu_links = [airflow_plugins_ml, astro_docs_ml, astro_guides_ml]

The astro_guides_ml is missing a ) after url='https://www.astronomer.io/guides/'