Fix wording for unselected checkbox state (#37415)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/37415

Previously, when focusing on an unchecked checkbox in React Native Paper with VoiceOver, it was announcing as "not selected".  iOS voiceover uses "unselected" instead ("not selected" is used on Android Talkback).  Update code to use "unselected".

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D45799922

fbshipit-source-id: 30f7813ed9f1fdad817e18b28c4fcf99cb4b884f

packages/react-native/React/Views/RCTView.m

@@ -310,7 +310,7 @@ RCT_NOT_IMPLEMENTED(-(instancetype)initWithCoder : unused)
         @"timer" : @"timer",
         @"toolbar" : @"tool bar",
         @"checked" : @"checked",
-        @"unchecked" : @"not checked",
+        @"unchecked" : @"unchecked",
         @"busy" : @"busy",
         @"expanded" : @"expanded",
         @"collapsed" : @"collapsed",