Building documentation>>cd doc: No such file or directory

[C4H7Cl2O4P]

Building documentation

$ pip install .[doc]
Processing /home/username/path/python-gsmmodem
WARNING: python-gsmmodem 0.9 does not provide the extra 'doc'
Collecting pyserial>=2.6
Using cached pyserial-3.5-py2.py3-none-any.whl (90 kB)
Building wheels for collected packages: python-gsmmodem
Building wheel for python-gsmmodem (setup.py) ... done
Created wheel for python-gsmmodem: filename=python_gsmmodem-0.9-py3-none-any.whl size=62008 sha256=b1fcde7af90eace2281df69b99d8809d1561028b6ddb18748ebe0efd4a193956
Stored in directory: /home/username/.cache/pip/wheels/bc/8f/1b/9383d2625e73f97c953c62f709cdd53ba679e2dbbe6ed1cb25
Successfully built python-gsmmodem
Installing collected packages: pyserial, python-gsmmodem
Successfully installed pyserial-3.5 python-gsmmodem-0.9

$ cd doc
bash: cd: doc: No such file or directory

What have I done wrong ? What should I do now ?