CustomPainterSemantics doc typo (flutter#137081)

Just a documentation typo, I think.

flutter#137080

Author: EnduringBeta

packages/flutter/lib/src/rendering/custom_paint.dart

@@ -293,7 +293,7 @@ abstract class CustomPainter extends Listenable {
 /// the accessibility of applications.
 ///
 /// Implement [CustomPainter.semanticsBuilder] to build the semantic
-/// description of the whole picture drawn by a [CustomPaint], rather that one
+/// description of the whole picture drawn by a [CustomPaint], rather than one
 /// particular rectangle.
 ///
 /// See also: