fixes misformating of docc term list output

[heckj]

resolves #772

The term list output originally followed the markdown format and embedded the trailing : in the bold section, but DocC doesn't recognize the list element as a term list if the : is embedded, only if it's trailing. This PR resolves that situation, and updates the associated snapshot tests.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[heckj]

@swift-ci test