Grammar in docs

Author: LeifAndersen

images-doc/images/scribblings/icons.scrbl

@@ -146,7 +146,7 @@ If you cannot, as with the Macro Stepper, send a thinner icon as the @racket[alt
 The backing scale of DrRacket icons.
 
 A backing scale of 2 means that the icon bitmap internally has two
-pixels per drawing unit, so it it renders well a double resolution,
+pixels per drawing unit, so it renders well at double resolution,
 such as Retina display mode for Mac OS X.
 
 @history[#:added "1.1"]}