[js_interop_unsafe] Update docs for []

This operator takes a Dart value but returns a JSAny?.

CoreLibraryReviewExempt: Doc-only change.
Change-Id: I67a004a7849f2c582efd479c2ccc2f36a07b4239
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/409480
Reviewed-by: Devon Carew <[email protected]>
Commit-Queue: Devon Carew <[email protected]>
Auto-Submit: Srujan Gaddam <[email protected]>
Commit-Queue: Srujan Gaddam <[email protected]>

Author: srujzs

sdk/lib/js_interop_unsafe/js_interop_unsafe.dart

@@ -39,7 +39,7 @@ extension JSObjectUnsafeUtilExtension on JSObject {
   external JSBoolean hasProperty(JSAny property);
 
   /// Shorthand helper for [getProperty] to get the value of the property key
-  /// [property] of this [JSObject], but takes and returns a Dart value.
+  /// [property] of this [JSObject], but takes a Dart value.
   JSAny? operator [](String property) => getProperty(property.toJS);
 
   /// The value of the property key [property] of this [JSObject].