Use summary instead of description

[cordula-grau]

Hi,

as the topic of the alert the summary and not the description should be used, I think.
The description is a more detailed explanation and may be to long for this.

https://github.com/bzon/prometheus-msteams/blob/28af43054976b1c027712ac273f100fa7431cbae/alert/teams.go#L126

Best
C.

[bzon]

@cordula-grau at the moment, users are already used to the current formatting so it doesn't make any sense to apply this change now. However, this can be solved in the future when I found the time to work on #12 which lets users use their own template to post to teams channel. :)