fix: change sfValidatorToReEnable to field code 21

[natenichols]

High Level Overview of Change

Corrects field code for sfValidatorToReEnable. fixes #127

Context of Change

Field code for sfValidatorToReEnable should be 21. See https://github.com/ripple/rippled/blob/develop/src/ripple/protocol/impl/SField.cpp/#L227

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation Updates
• [ ] Release

Note: I am personally of the opinion that this can wait to go into the next release, and does not merit its own re-release, as negativeUNL has not been enabled on mainnet yet. Open to other opinions from reviewers.

[natenichols]

@intelliot Yeah, something like that.

Similar to the idea y'all had a while back for generating the definitions.json from the rippled source code.