doc/examples/decoder_targeted: Disable error concealment after 20 frames

This allows testing EC and non EC. Avoids spending most time in EC on
high res samples and reduces the likelyhood of hitting timeouts

Fixes: Timeout in 467/fuzz-2-ffmpeg_VIDEO_AV_CODEC_ID_H263_fuzzer

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg

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