Hi,
The only damage you've done is dilute your coolant. Most pre-mixed Windshield Washer fluid is made of water, ethanol, or isopropanol, or ethylenglycol (usually labled Winter Mix).
Basically, you can just leave it in and you'll be OK. If obsessing over it, you can idle the car with the Bleeder Valve open and the alcohol solvent (ethanol, isopropanol) will just flash off, leaving you with a slightly diluted Coolant mix.
Next time you need a top-up, start by adding a Tsp or 2 of undiluted Coolant and you'll bring it back into balance.
While I wouldn't make a practice of doing this, the occasional inadvertent top-up with Windshield fluid won't harm it at all...
Happy Motoring!... Jim'99
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