you did not log out and log in ? It should be on your path to execute.
If not, pip installed executables in .local, so in a terminal:
~/.local/bin/jupyter-notebook
should start notebook
To be able to run jupyter notebook from terminal, you need to make sure that ~/.local/bin is in your path.
Do this by running export PATH=$PATH:~/.local/bin for your current session, or adding that line to the end of ~/.bashrc to make your changes last for future sessions (e.g. by using nano ~/.bashrc). If you edit ~/.bashrc you will need to log out and log back in to make see your changes take effect.