Hi Sara,
There really isn't a way to change that text. I have seen some tricks using Global Javascript or CSS to modify or hide specific text strings, but that would need to be done system wide; not just within a single course or assignment. My advice is to simply make sure in the written instructions for the Assignment to spell out the expectations that they must type their answer and not copy/paste if that is requirement. Canvas has that option I believe because depending on the length of an answer it's often easier to type it in another application first and then paste it. Not that it happens very often, but if someone is writing into a text entry box and they lose their internet connection before clicking Submit, they could lose all their work.
Rick
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