docs: Fixed resource/theme value syntax.

Within the "Resource values" sub-section within the "File Conventions"
section of the "App Manifest" page, changed the separator between the
name/value pair for resources and themes such that it is now
consistently a '/' character.

Bug: 27070041
Change-Id: Ie299fbb866b9de3afaffa8fb70e3ca3709c45735