This is NOT how you want to address this text change!! As with almost all extensions, the text is pulled from a language file. You don't need to dig into any PHP files here to make any such changes. Especially considered changing what you did will only affect that one location... not the calendar or other places.
The correct method to change is in the language file:
/language/en-GB/en-GB.com_dtregister.ini
This is the English file location. If you are using a different language file, go into the appropriate language folder. Search for the text you want to change, make your change and upload the modified file. Thanks!
ok cool i figured it would only change it on that page, but i was looking though the database and things trying to find where it stores all its terms and couldnt find it.