Typo: change "index exist" to "index exists" in error message

In the following error message, "exist" should be "exists":

cbq> create primary index on `travel-sample`;
{
"requestID": "7c0a9c93-b5be-41ef-bc9a-7637081c673a",
"signature": null,
"results": [
],
"errors": [

{ "code": 5000, "msg": "GSI CreatePrimaryIndex() - cause: Index #primary already exist." }

],
"status": "errors",
"metrics":

{ "elapsedTime": "7.461926ms", "executionTime": "7.421811ms", "resultCount": 0, "resultSize": 0, "errorCount": 1 }

}