output/wav: change default for scale factor from 0.0 to 1.0

output/wav: change default for scale factor from 0.0 to 1.0

The WAV output module supports an optional 'scale' factor, in its
absence the samples will pass unmodified. The builtin help text is
unexpected, and reads:

  $ sigrok-cli -O wav --show
  ...
  Options:
    scale: Scale values by factor (default 0.0)

Setup a default scale factor of 1.0, which results in identical
behaviour and better reflects what is happening.