Fix CER calculation in chapter 5

[TomSteer1]

The calculation for the CER in Chapter 5 does not take into account the spaces, unlike the implementation in the evaluate library or other implementations.

>>> from evaluate import load
>>> cer_metric = load("cer")
>>> reference = "the cat sat on the mat"
>>> prediction = "the cat sit on the"
>>> cer_metric.compute(references=[reference],predictions=[prediction])
0.22727272727272727
>>>

>>> from jiwer import cer
>>> reference = "the cat sat on the mat"
>>> prediction = "the cat sit on the"
>>> cer(reference,prediction)
0.22727272727272727

[Deep-unlearning]

thanks for the fix!

[HuggingFaceDocBuilderDev]

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