Fix typo in RefreshIndicator constructor API doc

packages/flutter/lib/src/material/refresh_indicator.dart

diff --git a/packages/flutter/lib/src/material/refresh_indicator.dart b/packages/flutter/lib/src/material/refresh_indicator.dart
index b2b95ad..a03b736 100644
--- a/packages/flutter/lib/src/material/refresh_indicator.dart
+++ b/packages/flutter/lib/src/material/refresh_indicator.dart
@@ -91,7 +91,7 @@
   /// The [semanticsLabel] is used to specify an accessibility label for this widget.
   /// If it is null, it will be defaulted to [MaterialLocalizations.refreshIndicatorSemanticLabel].
   /// An empty string may be passed to avoid having anything read by screen reading software.
-  /// The [semanticsValue] may be used to specify progress on the widget. The
+  /// The [semanticsValue] may be used to specify progress on the widget.
   const RefreshIndicator({
     Key key,
     @required this.child,