Americanise spellings (#33323)

Updates documentation and non-public API to use American spellings for
consistency with the rest of the codebase.

No changes to behaviour... other than how it's spelt.
diff --git a/examples/layers/widgets/spinning_mixed.dart b/examples/layers/widgets/spinning_mixed.dart
index 9795d64..b7be577 100644
--- a/examples/layers/widgets/spinning_mixed.dart
+++ b/examples/layers/widgets/spinning_mixed.dart
@@ -7,7 +7,7 @@
 
 import '../rendering/src/solid_color_box.dart';
 
-// Solid colour, RenderObject version
+// Solid color, RenderObject version
 void addFlexChildSolidColor(RenderFlex parent, Color backgroundColor, { int flex = 0 }) {
   final RenderSolidColorBox child = RenderSolidColorBox(backgroundColor);
   parent.add(child);
@@ -15,7 +15,7 @@
   childParentData.flex = flex;
 }
 
-// Solid colour, Widget version
+// Solid color, Widget version
 class Rectangle extends StatelessWidget {
   const Rectangle(this.color, { Key key }) : super(key: key);