WordPress uses Dashicons. Basically you would replace your code with this:
add_menu_page('Cool Plugin Page', 'Cool Plugin', 'manage_options', 'my_main_menu','my_main_plugin_page', 'dashicons-admin-post', '');
WordPress uses Dashicons. Basically you would replace your code with this:
add_menu_page('Cool Plugin Page', 'Cool Plugin', 'manage_options', 'my_main_menu','my_main_plugin_page', 'dashicons-admin-post', '');