Fix metadata_per_document value in total_metadata calculation

The [Memory quota](http://docs.couchbase.com/couchbase-manual-2.2/#memory-quota) section contains the **metadata_per_document** constant value **56**, but the calculation following that for **total_metadata** is:

`1,000,000 * (100 + 120) * (2) = 440,000,000`

where the constant value `120` for **metadata_per_document** is actually the old Couchbase Server 1.8.1 value of **120**, so the calculation should really reflect the value **56** instead for Couchbase Server version 2.2.0 documentation:

`1,000,000 * (100 + 56) * (2) = 312,000,000`