wrong unit-test in slate-history

**Problem**
packages/slate-history/test/undo/delete_backward/block-text.tsx

It seems should be 
```jsx
export const run = editor => {
   Transforms.delete(editor, {reverse: true});
}
```
rather than
```jsx
export const run = editor => {
  Transforms.delete(editor)
}
```
add reverse option, and it could pass