Added missing comma in sample response json for list of boards and a single board

What does this MR do?

1. In the sample response json for getting a single project issue board, and in the one for a list of issue boards for a project, there are a set of commas missing under the milestone: key. Added those.

2. In the sample response json for getting a single project issue board, the name key has an extraneous : in its keyname. Removed it.

These a very small change, but I found it a little troubling when I tried to copy these for fixtures while working on a ruby wrapper for Gitlab API https://github.com/NARKOZ/gitlab. Hence raising this MR.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer