Small fixes and typos to some code blocks

[saifm-ai]

Made the following changes:

• docs/docs/getting-started/4-dog-on-couch.md: we should be using gl.get_or_create_detector() method instead of gl.get_detector(), throws an error if the detector was not previously created
• docs/docs/getting-started/5-streaming.md: added brew install ffmpeg, else the script fails
• docs/docs/getting-started/5-streaming.md: added: chmod +x get_latest_frame.sh else there's a permissions error
• docs/docs/getting-started/5-streaming.md: we should be using gl.get_or_create_detector() method instead of gl.get_detector(), throws an error if the detector was not previously created
• docs/docs/getting-started/5-streaming.md: Typo here, to run the script, it should be
  python streaming_alert.py --video-id=<VIDEO_ID> --detector-name=<DETECTOR_NAME> --query=<QUERY IN QUOTATION MARKS>

More info: https://docs.google.com/spreadsheets/d/1uQD0Ojp0Mma5FEznZ9_Jz8Nx_MwxT15lZDw2rjrYGvU/edit#gid=0

[saifm-ai]

@blaise-muhirwa @robotrapta

[mjvogelsong]

Thanks for the improvements!