Fix RingBuilder._build_max_replicas_by_tier docstring

The current docstring doesn't include zones, and the order of the
entries is not up to date with the current code. Let's fix this.

Change-Id: Ibabd79427b83d9e8c86b2caeb93dee219c8274c0
Merge "Fix RingBuilder._build_max_replicas_by_tier docstring"