fix shebang

[santarl]

• fixed a typo in the shebang (script can now be called directly with ./facer_rgb.py instead of python3 ./facer_rgb.py)

• changed shebang from #!/usr/bin/python3 to #!/usr/bin/env python3 to allow the selection of python3 shell to env instead of hardcoding it (helpful if python3 is installed in directories other than /usr/bin/)

[JafarAkhondali]

Sorry for the delay, it worked great