fixed formatting of logging command
fixed formatting of logging command

This avoids this error when running the redis spider:

```shell
--- Logging error ---
Traceback (most recent call last):
      File "/usr/lib/python3.5/logging/__init__.py", line 980, in emit
          msg = self.format(record)
        File "/usr/lib/python3.5/logging/__init__.py", line 830, in
      format
          return fmt.format(record)
        File "/usr/lib/python3.5/logging/__init__.py", line 567, in
      format
          record.message = record.getMessage()
        File "/usr/lib/python3.5/logging/__init__.py", line 330, in
      getMessage
          msg = msg % self.args
      ValueError: incomplete format
```

more traceback:

```shell
Message: "Reading start URLs from redis key '%(redis_key)s' (batch size:
%(redis_batch_size)s, encoding: %(redis_encoding)"
Arguments: {'redis_encoding': 'utf-8', 'redis_batch_size': 16,
    'crawler': <scrapy.crawler.Crawler object at 0x7fdd0528bc50>,
    'redis_key': 'hiplead:start_urls', 'settings':
        <scrapy.settings.Settings object at 0x7fdd00786358>,
    'start_urls': []}
```

rolando/scrapy-redis,darkrho/scrapy-redis