fix section size from numberOfSectionsInTableView


example code:
```ruby
@promotion_table_data.data = [
  { title: 'section 1', cells: [ { title: 'cell 1-1' }, { title: 'cell 1-2' } ], },
  { title: 'section 2', cells: [ { title: 'cell 2-1' }, { title: 'cell 2-2' } ], },
  # ....
  { title: 'section 8', cells: [ { title: 'cell 8-1' }, { title: 'cell 8-2' } ], },
  { title: 'section 9', cells: [ { title: 'cell 9-1' }, { title: 'cell 9-2' } ], },
]

Array(@promotion_table_data.data).length
# => 3

@promotion_table_data.data.size
# => 9
```

and sorry, I don't know how to write tests in this case.