Making a div before the calender to include content/text will fix the positioning problem, but still doesn't really help if the shortcode is used in the middle of a page.
The problem occurs because the shortcode function is using 'echo', rather than 'return'. Here is a fix which will position the calender exactly in the corrct position by 'returning' the whole function at the end.
Open appointment-calendar-shortcode.php, near the top of the page (around line 5) is the function 'appointment_calendar_shortcode()'. Paste the code in bold shown below at the very start of the function.
Now go to the very bottom of the page and find the closing bracket for the function (//end of short code function). Paste the code shown below in bold just BEFORE the closing bracket.
$apcalpositionfix = ob_get_clean();
Save the changes and upload the file to your FTP. Refresh the page and the calender should be in the correct position.