Sidekiq::MemoryKiller: Add missing space in log message

What does this MR do?

The sidekiq memory killer prints warnings like

gitlab-sidekiq[8245]: 2017-09-27T11:31:43.192Z 8245 TID-ov9l6b4e0 WARN: this thread will shut down PID 8245 - Worker ProjectWebHookWorker - JID-dd3b1c23a653deec5e575ab4in 900 seconds

There is a space missing between the JID and 'in'.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?