A bug in initialize of Map

rgss3.rb
```
class RPG::Map
  def initialize(width, height)
  ...
    @battleback_floor_name = ''
    @battleback_wall_name = ''
  ...
  attr_accessor :battleback1_name
  attr_accessor :battleback2_name
```
The initialized variable names should be the same as below.
Otherwise it will result in an inability to write back, the `battleback1_name` and `battleback2_name` will be nil.