If you edit core files they can disappear when you update Joomla. To edit this text you should use the Language Overrides feature already within Joomla.
Go to Extensions > Language(s) and select Overrides. Click on New in the upper left. On the right hand side you enter the text you are looking for. In your case Don't have an account? and click Search. Click on the search result that pops up. On the left hand side it will display the text you want to change. Simply change it to whatever you desire and click Save and Close.
Now when you update Joomla in the future you don't have to worry about losing your changes. You also have the option to edit or delete the override you have made anytime.
See https://docs.joomla.org/J3.x:Language_Overrides_in_Joomla
