Unless you are in group web, you do not have permissions to write that file. If you own the system, you can use sudo
to accomplish what you need. However, if you had sudo access, you probably wouldn't be asking.
Since you own the directory, you can delete the file. So one workaround would be: save the text of the file somewhere else (in another window). Delete the file. Recreate the file in an editor, pasting in the text. Save. That should do it. However, it's possible since this is a script that doing this would break the script (it might need to run as user 'web').
The safest alternative is to ask the system administrator to add you to the group "web", or else to have him or her change the group of the file to a group that you are in.