Adjust wording of stale issue message

[pyrmont]

This improves the wording of the message.

[ashmaroli]

@pyrmont I'm not well-versed with the configuration params of Stalebot. But Ideally the bot shouldn't mark an issue as stale if the issue is labelled with has-pull-request or has-PR or PR-attached, etc..

[ashmaroli]

Ah! I commented too soon.. That's exactly what the exemptLabels: list is for..

[pyrmont]

No, it's a good point. To be honest, using Probot is a bit of an experiment and I'm sort of learning as I go.

I like the idea of tagging issues that are related to an active PR with a label that prevents the issue being marked as stale. I'll be going through the issues marked later tonight and will add that label (and include it in the list of exemptions).