Create signed tags when tagging a release

Change-Id: I686e5028d354509b34cfbfdba6550139b45a6329
diff --git a/tagRelease.bash b/tagRelease.bash
index d639f70..0ac02e4 100644
--- a/tagRelease.bash
+++ b/tagRelease.bash
@@ -48,6 +48,6 @@
 
 commitMessage1="$nextVersion"
 commitMessage2="Changelog: $GITILES_REPO_URL/+log/refs/tags/$prevVersion..refs/tags/$nextVersion"
-git tag -a $nextVersion -m "$commitMessage1" -m "$commitMessage2"
+git tag -s $nextVersion -m "$commitMessage1" -m "$commitMessage2"
 
 echo "Tag created. Now run \`git push --tags\` to push the tags to the server."