monitoring: Fix "10% OSDs down" alert description

monitoring: Fix "10% OSDs down" alert description

The alert was triggered when less than 90% of OSDs were _up_, but then the
description took that value and described it as the percentage of OSDs being
_down_. So with 12% of OSDs down, the alert description would read:

```
88% or 88 of 100 OSDs are down (>=10%).
```

which can be panic-inducing.

This commit changes the alert expression to actually compute the ratio of OSDs
being down, which makes the correct value appear in the description.

Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>