Fix showCupertinoModalPopup documentation comment typo (flutter#52995)

Author: mono0926

packages/flutter/lib/src/cupertino/route.dart

@@ -905,7 +905,7 @@ class _CupertinoModalPopupRoute<T> extends PopupRoute<T> {
 /// popup to the [Navigator] furthest from or nearest to the given `context`. It
 /// is `false` by default.
 ///
-/// The `semanticsDismissble` argument is used to determine whether the
+/// The `semanticsDismissible` argument is used to determine whether the
 /// semantics of the modal barrier are included in the semantics tree.
 ///
 /// The `builder` argument typically builds a [CupertinoActionSheet] widget.