fix a typo (flutter#150682)

fix typo

*List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

Author: foxmind1

packages/flutter/lib/src/semantics/semantics_event.dart

@@ -159,7 +159,7 @@ class TapSemanticEvent extends SemanticsEvent {
 /// An event to move the accessibility focus.
 ///
 /// Using this API is generally not recommended, as it may break a users' expectation of
-/// how a11y focus works and therefore should be just very carefully.
+/// how a11y focus works and therefore should be used very carefully.
 ///
 /// One possible use case:
 /// For example, the currently focused rendering object is replaced by another rendering