config.sample.yaml: alertmanager: update example (text|html)_template

[herbetom]

I've updated the (text|html)_template within in the sample config file and made the following changes:

• better readability: This helps other's since it allows custom modifications more easily.
• introduced the severity info: it's blue and probably quite common (hoewer other's might use none but that should be a easy fix for them)
• dropped the FIRING - prefix: With limited space in Pop-ups i think the first few characters are too important as to waste them with kinda redundant information. [RESOLVED] and everthing else should be enough to distinguish between a firing alert and a resolved one.

Since it's only an example config i don't really see an issue with changing it. As long as others agree that my version is better. ;)