title: fix confusing bleu metric example
body: **Is your feature request related to a problem? Please describe.**
  I would like to see the example in "Metric Card for BLEU" changed.
  The 0th element in the predictions list is not closed in square brackets, and the 1st list is missing a comma.
  The BLEU score are calculated correctly, but it is difficult to understand, so it would be helpful if you could correct this.
  ```
  >> predictions = [
  ...     ["hello", "there", "general", "kenobi",       # <- no closing square bracket.
  ...     ["foo", "bar" "foobar"]                              # <- no comma between "bar" and "foobar"
  ... ]
  >>> references = [
  ...     [["hello", "there", "general", "kenobi"]],
  ...     [["foo", "bar", "foobar"]]
  ... ]
  >>> bleu = datasets.load_metric("bleu")
  >>> results = bleu.compute(predictions=predictions, references=references)
  >>> print(results)
  {'bleu': 0.6370964381207871, ...
  ```
  
  **Describe the solution you'd like**
  ```
  >> predictions = [
  ...     ["hello", "there", "general", "kenobi",       # <- no closing square bracket.
  ...     ["foo", "bar" "foobar"]                              # <- no comma between "bar" and "foobar"
  ... ]
  # and
  >>> print(results)
  {'bleu':1.0, ...
  ```
  