Fix readme

[InquestGeronimo]

fixing indentation in YAML code block

[dbarbuzzi]

The indentation is still incorrect for the YAML block; the lines under each of the - task: […] lines are still indented too much. The block in the Markdown source should be:

```yaml
models:
    - task: question_answering
      model_path: zoo:nlp/question_answering/bert-base/pytorch/huggingface/squad/base-none
      batch_size: 1
      alias: question_answering/base
    - task: question_answering
      model_path: zoo:nlp/question_answering/bert-base/pytorch/huggingface/squad/pruned_quant-aggressive_95
      batch_size: 1
      alias: question_answering/pruned_quant
```

Basically, the indentation of the first mapped value has to be matched by the remaining ones exactly/visually (ignoring the - used to denote a list item).