docs: Fix a few typos

[timgates42]

There are small typos in:

• docs/concurrency-challenges.rst
• examples/ptpython_config/config.py
• ptpython/completer.py
• ptpython/history_browser.py
• ptpython/key_bindings.py
• ptpython/repl.py

Fixes:

• Should read returns rather than retuns.
• Should read parentheses rather than parethesis.
• Should read output rather than ouptut.
• Should read navigation rather than navigaton.
• Should read executor rather than excecutor.
• Should read depending rather than deponding.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[jonathanslenders]

Thanks!