chore: update Error message for bad bigtable.env variable (#91)

At some point, the allowed settings were changed from "prod" and "emulator" to "cloud" and "emulator".
This change updates the error message associated to the illegal argument exception.

Author: sid-dinesh94

google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/test_helpers/env/TestEnvRule.java

@@ -103,7 +103,7 @@ protected void before(Description description) throws Throwable {
       default:
         throw new IllegalArgumentException(
             String.format(
-                "Unknown env: %s. Please set the system property %s to either 'emulator' or 'prod'.",
+                "Unknown env: %s. Please set the system property %s to either 'emulator' or 'cloud'.",
                 env, ENV_PROPERTY));
     }
     testEnv.start();