docs:monitor: Fill in inventory attribute details

The `inventory` attribute contains a string of the relative path under
`/xyz/openbmc_project/inventory` where the fan's OperationalStatus
interface exists.

Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
Change-Id: I6be09429c940fadb28d194c2519cf5afb3d502d9
1 file changed