Add a short code to a plugin

Here is the code that you need to pull content from a wordpress page you will require the id number of the page and add it into the shortcode.
[example page_id=”21″][/example]

function example_shortcode( $atts, $content = null) {

    extract( shortcode_atts( array(
                'page_id' => ''
            ), $atts 
        ) 
    );
    // Will display our page content of the shortcode need a number of the page id

      $post = get_post($page_id); 
      $content = apply_filters('the_content', $post->post_content); 
      echo $content;  

}
add_shortcode('example', 'example_shortcode'); 

//[example page_id="page id number here"][/example]