Retrieving deleted text within a note on my iPhone
iPhones have an Undo function. By default, to trigger the undo option you need to give your phone a quick shake. In your case it may be too late to use it. 

To find out, go into your note and then give the phone a quick shake. If it works you will get an option such as the one below:



Obviously, then you would tap on Undo to undo the last typing action.