Updated doctest's output to reflect error message found in summer 2019.

• When running doctest for parse('invalid'):
  • error message encountered during lesson prep for soft 162 was different than error message in doctest
  • changed doctest to reflect current error message
• Possible discussion point for future is that error messages may change in future versions of Python, causing false-failures of tests

mentioned in commit c1c69b2e

merged