Docs: Clarify how many zones can fail

[osg-grafana]

@56quarters shares that the formula and explanation for how many zones can fail is very confusing at https://grafana.com/docs/mimir/latest/operators-guide/configure/configure-zone-aware-replication/#minimum-number-of-zones.

... based on these docs, it seems like a replication factor of 2 is completely fine and will tolerate a zone being down with no write outage (which it will NOT)

"If there are no more than floor(replication factor / 2) zones with failing replicas, reads and writes can withstand zone failures."

... if replication factor = 2, the value of that is floor(2 / 2) which implies it will handle 1 zone being down

... no more than translates to <= when really it should be fewer than which is <