docs: update CONTRIBUTING.md & config.example.toml

[SteveLauC]

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md

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What does this PR do

1. Update the example cases given in CONTRIBUTING.md where arguments in addition to ctx are required

   The old example is not suitable as cleanup is included in ExecutionContext, we don't need that cleanup argument, the related code will be fixed in refactor: make update fn take &ExectionContext & put update fn together #457

2. Fix some typos in CONTRIBUTING.md

3. Uncomment [include] section in config.example.toml