[css-backgrounds-3][editorial] Fixed typo

Author: SebastianZ

css-backgrounds-3/Overview.bs

@@ -3562,7 +3562,7 @@ Shadow Shape, Spread, and Knockout</h4>
 	from the border-box (padding-box) radii by adding (subtracting)
 	the [=spread distance=] (and flooring at zero).
 	For outer shadows, the [=border-radius=] is then [=outset-adjusted border radius|adjusted=], independently in each dimension,
-	to preseve the sharpness of rounded corners.
+	to preserve the sharpness of rounded corners.
 
 	The 'border-image' does not affect the shape of the box-shadow.