Correct errors preventing proper display of Riak CS GC document

Correct some issues with malformed version guards preventing the
display of most of the information in the Riak CS garbage collection
document for the 1.5.0 version.
These two can actually be combined, since they start the same:

```
`batch` | Manually start garbage collection for a batch of eligible objects.{{#1.5.0+}} This command takes an optional argument to indicate a leeway time other than the currently configured `leeway_seconds` time for the batch.{{/1.5.0+}}
```