Fix gsi-pubsign-release.sh rsync command

Add trailing / to rsync source directory, this prevents rsync from
creating an additional level of directory under the destination.

For example, if `tree src` outputs:
  src
  └── file

then:
$ rsync src dest   # creates dest/src/file
$ rsync src/ dest  # creates dest/file

Exempt-From-Owner-Approval: Hotfix / doesn't affect build
Bug: 149803895
Test: ./gsi-pubsign-release.sh aosp_arm64 aosp_arm64_pubsign \
        aosp_arm64-artifact_path aosp_arm64_pubsign-artifact_path

Change-Id: I90a80a7afdf6f71db592599c6a645554cf830309
1 file changed