prefer "SAR" over "PAR" in av_dump_format()

prefer "SAR" over "PAR" in av_dump_format()

"SAR" (Sample Aspect Ratio) is globally preferred over "PAR" (Pixel
Aspect Ratio), although the two terms share the same semantics.

For example the corresponding AVStream field is called
sample_aspect_ratio, and libavfilter has a filter named setsar.

Therefore prefer the term "SAR" over "PAR" in the
libavformat/utils.c:dump_stream_format() and avcodec_string() output
for avoiding confusion.