Fixed README syntax error for #allow_destroy in :accepts_nested_attributes_for matcher

The example in the README had an error:
```ruby
it { should accept_nested_attributes_for(:mirrors).allow_destroy }
```
it throws an error `ArgumentError: wrong number of arguments (0 for 1)`

Since the signature of the `allow_destroy` now requires a parameter and can test both cases, the README should be updated to the following:
```ruby
it { should accept_nested_attributes_for(:mirrors).allow_destroy(true) }
```