Fix typo in normalize tensor function

[philiporlando]

@Athospd, I noticed a typo within the definition of the normalize() function here. Python's dot notation was used instead of R's $ extract operator.

The current version produces this error:

normalize(waveform)
# Error in tensor.mean() : could not find function "tensor.mean"

Changing tensor.mean() to tensor$mean() resolves this issue.

[Athospd]

Thanks, @philiporlando