Fix minor spelling error (flutter#143541)

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packages/flutter/lib/src/rendering/box.dart

@@ -1989,7 +1989,7 @@ abstract class RenderBox extends RenderObject {
             'RenderBox.size accessed beyond the scope of resize, layout, or '
             'permitted parent access. RenderBox can always access its own size, '
             'otherwise, the only object that is allowed to read RenderBox.size '
-            'is its parent, if they have said they will. It you hit this assert '
+            'is its parent, if they have said they will. If you hit this assert '
             'trying to access a child\'s size, pass "parentUsesSize: true" to '
             "that child's layout().",
           );