Fix README so it doesn't talk about XCTest "bug"

It's not a bug. It's the design.

README.md

@@ -323,9 +323,9 @@ deallocated so that the swizzling is undone.
 
 In the example above, `mockUserDefaultsClass` will go out scope and be
 destroyed. But what if you kept it in the test fixture, as an ivar or a
-property? Due to a bug in XCTest, it won't be implicitly destroyed. You need to
-explicitly set it to nil in `-tearDown`, or the swizzling will bleed over to
-your other tests, compromising their integrity.
+property? According to XCTest's design, it won't be implicitly destroyed.
+You need to explicitly set it to nil in `-tearDown`, or the swizzling will
+bleed over to your other tests, compromising their integrity.
 
 If you need more control over when the swizzling is undone, call
 `stopMocking(…)` on the mock class.