Update annotation files for SSv2 keyframe dataset. Webm to mp4

[martinwholtmon]

Because we fixed the error when we compile the videos back into .mp4 instead of webm, we must update the csv files as well.

Should only convert file extension for webm, and notify the user after the training has finished with how to fix the csv files:
find . -name '*.csv' -exec sed -i -e 's/.webm/.mp4/g' {} \;

[martinwholtmon]

Add a note to the documentation, it is only for SSv2 when compiling frames into a new video.