osxcoreaudio: fix typo

kAudioFormatFlagIsSignedInteger is a format flags

Closes #394
6 jobs for osxcoreaudio in 34 minutes and 7 seconds (queued for 61 minutes and 33 seconds)
Status Job ID Name Coverage
  Preparation
passed #65406
gst indent

00:00:16

passed #65405
manifest

00:00:18

 
  Build
passed #65407
build fedora x86_64

00:03:43

 
  Test
passed #65408
check fedora

00:15:53

 
  Full Builds
passed #65409
build android arm64 api28

00:02:29

passed #65410
build cerbero fedora x86_64

00:11:25