chore: fix distribution_name in .repo-metadata (googleapis#24)

Fixes googleapis#13

Author: chingor13

.repo-metadata.json

@@ -6,5 +6,6 @@
   "release_level": "ga",
   "language": "java",
   "repo": "googleapis/java-spanner-jdbc",
-  "repo_short": "java-spanner-jdbc"
-}
+  "repo_short": "java-spanner-jdbc",
+  "distribution_name": "com.google.cloud:google-cloud-spanner-jdbc"
+}