translate text: "subnet"/"subnet details"

When trying to create or update a subnet from the Networks
details page, the tab titles 'Subnet' and 'Subnet details'
are not showing as translated. This patch add the missing
underscore.

Change-Id: I48cd47e7ead3717d96e4808f41bf876a567c8052
Closes-Bug: #1254026
Merge "translate text: "subnet"/"subnet details""