Make audio format consistent in decoder and reader

[gBillal]

I updated the audio sample format in reader to accept the ffmpeg naming too.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 77.65%. Comparing base (029ee0d) to head (2b202ba).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/xav/frame.ex	20.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #33       +/-   ##
===========================================
- Coverage   88.09%   77.65%   -10.44%     
===========================================
  Files           6        7        +1     
  Lines          84       94       +10     
===========================================
- Hits           74       73        -1     
- Misses         10       21       +11     

Files with missing lines	Coverage Δ
lib/xav/decoder.ex	80.00% <ø> (ø)
lib/xav/reader.ex	91.42% <ø> (ø)
lib/xav/frame.ex	42.85% <20.00%> (ø)

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[gBillal]

I'm merging this, if you have any issue, I'll fix them.