Change display of attendee limit text in Event Espresso 4's calendar tooltip

calendar_tooltip.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter(
    'FHEE__EE_Calendar__get_calendar_events__tooltip_reg_btn_html',
    'my_custom_calendar_available_spaces',
    10,
    3 
);
function my_custom_calendar_available_spaces(
    $original_output, 
    $event,
    $datetime
) {
    $text = 'Spaces Available: ' . $datetime->total_tickets_available_at_this_datetime();
    $spaces = '<p class="attendee_limit_qtip">' . $text . '</p>';
    return $spaces . $original_output;
}