I assume you're in Code View, typing something like:
and it's changing it to:
Is that correct?
If so, that is part of the jQuery code hinting. You can disable all of the jQuery code hinting functionality by going to Preferences > Code Hinting, and then uncheck jQuery Code Hints in the list.
However, note that this will disable all code hinting for jQuery. I'm not aware of any way to disable only the auto-append of the single quote.
If being able to disable this particular functionality without disabling all jQuery code hinting is something you would like to be able to do, be sure to send this request directly to the Dreamweaver development team: http://adobe.ly/DWwish