Correct error message for unset GOOGLE_CLOUD_PROJECT. (#2448)

Author: dzlier-gcp

spanner/cloud-client/src/main/java/com/example/spanner/SpannerSample.java

@@ -1667,8 +1667,8 @@ public static void main(String[] args) throws Exception {
       String clientProject = spanner.getOptions().getProjectId();
       if (!db.getInstanceId().getProject().equals(clientProject)) {
         System.err.println(
-            "Invalid project specified. Project in the database id should match"
-                + "the project name set in the environment variable GCLOUD_PROJECT. Expected: "
+            "Invalid project specified. Project in the database id should match the"
+                + "project name set in the environment variable GOOGLE_CLOUD_PROJECT. Expected: "
                 + clientProject);
         printUsageAndExit();
       }