Fix api docs for logging endpoint

[nlochschmidt]

Description

• Corrects indentation for the /system/logs endpoint (summary,...)
• Fixes schema validation for LogEntry

Motivation and Context

• I have raised an issue to propose this change (required)
• My issue has received approval from the maintainers or lead with the design/approved label

Fixes errors in the api docs as raised in #1287

How Has This Been Tested?

Loaded the API Docs into Swagger Editor and they validate without errors.

Types of changes

• 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 change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[nlochschmidt]

I am not sure why the build has failed.
@alexellis could you have a look? https://travis-ci.org/openfaas/faas/builds/574302332

The build ran through fine on my fork of faas 🤷‍♂(see https://travis-ci.org/nlochschmidt/faas/builds/574300172)

[alexellis]

I can't tell why, but it's unrelated to your change.

[alexellis]

LGTM

[nlochschmidt]

Thanks