Fix grammar in #convert-destpolar

Author: tiaanl

css-color-4/Overview.bs

@@ -4236,7 +4236,7 @@ with white point |dest-white|:
 	using a linear Bradford [=chromatic adaptation transform=],
 	and let this be the new |xyz|.
 
-	<li id="convert-destpolar">If |dest| is a a <a>cylindrical polar color</a> representation,
+	<li id="convert-destpolar">If |dest| is a <a>cylindrical polar color</a> representation,
 	let |dest-rect| be the corresponding  <a>rectangular orthogonal color</a> representation.
 	Otherwise, let |dest-rect| be |dest|.