presets: spelling error in libvpx 1080p50_60

Signed-off-by: Janne Grunau <janne-libav@jannau.net>

presets/libvpx-1080p50_60.avpreset

@@ -1,6 +1,6 @@
 g=120
 lag-in-frames=25
-deafline=good
+deadline=good
 cpu-used=0
 profile=1
 qmax=51