Typo in Azure DevOps config struct

[wiggin77]

Typo in the json tag here: https://github.com/mattermost/mattermost-plugin-azure-devops/blob/2d2f0ab473ddfcbfe7873027595d392a4c4e6959/server/config/config.go#L24

As a result the field name written by json.Marshal is capitalized whereas the schema defined in plugin.json expects a lower-case initial. This will probably cause issues in the system console.

Jira: https://mattermost.atlassian.net/browse/MM-51248