minor fixes in the example notebooks

[7anooch]

Two things (in two separate commits):

• in examples/make_trails.ipynb the last two cells referenced the wrong array variable (video_array instead of time_encoded_array), I just fixed that. Now the notebook runs without error from start to end.

• in examples/add_timestamp.ipynb the video path being read was an absolute path to a file on the repo author's computer. Changed that to the same sample video used in make_trails.ipynb