Fix expires length typo in mod assign edit feedback stamps plugin files

Requirements Ghostscript (pathtogs) A course with a student Testing Login as a teacher Create an assignment Open your browser developer tools and ensure that the "Network" tab is collecting data Press the "Grade submissions" button Using the EditPDF tools add some stamps to the (empty) page Inspect the stamps on the page using your browser tools. The URL should look similar to: /pluginfile.php/1/assignfeedback_editpdf/systemstamps/8c96a486d5801e0f4ab8c411f561f1c687e1f865/cross.png Open the network tab and filter by systemstamps to find the stamps Select one of the stamps and look at the Response headers Confirm that the Expires section is in the far future Confirm that the max-age is a large number (like 7776000 ) Confirm that an " etag " header is set *Confirm that the Content-Disposition includes " inline "