[Bugfix:Developer] formatting error (Submitty#515)

The bottom of php.md looks like this:

![image](https://github.com/Submitty/submitty.github.io/assets/47994802/940822e3-b725-49c6-b947-8c2a4f420b3d)

Added the last back tick to make it work properly

Author: mahi-pas

_docs/developer/software_and_system_design/coding_style_guide/php.md

@@ -123,4 +123,4 @@ an error instead. To do this, place at the top of the file:
 <?php
 
 declare(script_types=1);
-``
+```