Output of the example for grouping in docs is wrong

Hello.

In the Group Commands section (https://github.com/google/python-fire/blob/master/docs/guide.md#grouping-commands), the example code is

```python
class IngestionStage(object):

def run(self):
return 'Ingesting! Nom nom nom...'

class DigestionStage(object):

def run(self, volume=1):
return ' '.join(['Burp!'] * volume)

def status(self):
return 'Satiated.'

class Pipeline(object):

def __init__(self):
self.ingestion = IngestionStage()
self.digestion = DigestionStage()

def run(self):
self.ingestion.run()
self.digestion.run()
return 'Pipeline complete'

if __name__ == '__main__':
fire.Fire(Pipeline)
```

and according to the guide

```bash
$ python example.py run
Ingesting! Nom nom nom...
Burp!
```

However, the output is wrong. It should be

```bash
$ python example.py run
Pipeline complete
```

This can be easily fixed.
However, in order to demonstrate the pipeline functionality, I would suggest to change the example to

```python
class IngestionStage(object):

def run(self):
return 'Ingesting! Nom nom nom...'

class DigestionStage(object):

def run(self, volume=1):
return ' '.join(['Burp!'] * volume)

def status(self):
return 'Satiated.'

class Pipeline(object):

def __init__(self):
self.ingestion = IngestionStage()
self.digestion = DigestionStage()

def run(self):
ingestion_output = self.ingestion.run()
digestion_output = self.digestion.run()
return ingestion_output + digestion_output

if __name__ == '__main__':
fire.Fire(Pipeline)
```

and accordingly the output should be

```bash
$ python example.py run
Ingesting! Nom nom nom...Burp!
```

I would like to know which change @dbieber would like.
Thank you.

閱讀 docs/guide.md 中的 Group Commands 區段，並將文件中的 Python 範例與執行 python example.py run 的輸出進行比較。向 maintainer 確認範例應顯示 Pipeline complete，還是應示範中間階段的輸出，然後一致地更新程式碼和預期輸出。

由索引模型根據 Issue 內容生成。