DOC: Fix spaces and brackets in ValueError message for option_context.
Add a missing space and changed a parenthesis to a bracket in the message of the ValueError raised whenever  the `args` in `option_context(*args)` don't meet requirements.
'Need to invoke asoption_context(pat, val, [(pat, val), ...)).'
->
'Need to invoke as option_context(pat, val, [(pat, val), ...]).'

- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
