doc: fix bistream typo

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/libav-merge.txt

@@ -97,7 +97,7 @@ Stuff that didn't reach the codebase:
 - VAAPI VP8 decode hwaccel (currently under review: http://ffmpeg.org/pipermail/ffmpeg-devel/2017-February/thread.html#207348)
 - Removal of the custom atomic API (5cc0057f49, see http://ffmpeg.org/pipermail/ffmpeg-devel/2017-March/209003.html)
 - Read aac_adtstoasc extradata updates from packet side data on Matroska once mov and the bsf in question are fixed (See 13a211e632 and 5ef1959080)
-- new bistream reader (see http://ffmpeg.org/pipermail/ffmpeg-devel/2017-April/209609.html)
+- new bitstream reader (see http://ffmpeg.org/pipermail/ffmpeg-devel/2017-April/209609.html)
 - use of the bsf instead of our parser for vp9 superframes (see fa1749dd34)
 
 Collateral damage that needs work locally: