flvmux: Fix scale of time values in warning message

8 jobs for flvmux-msg in 34 seconds (queued for 24 seconds)
Status Job ID Name Coverage
  Preparation
canceled #174235
gst indent

00:00:11

canceled #174234
manifest

00:00:34

 
  Build
canceled #174236
build fedora x86_64

 
  Test
canceled #174237
check fedora

 
  Full Builds
canceled #174239
build cerbero android universal

canceled #174240
build cerbero cross win32

canceled #174241
build cerbero cross win64

canceled #174238
build cerbero fedora x86_64