Fix docs in metastore databricks_grants example (#3239)

Author: donatasm

docs/resources/grants.md

@@ -35,6 +35,7 @@ You can grant `CREATE_CATALOG`, `CREATE_CONNECTION`, `CREATE_EXTERNAL_LOCATION`,
 
 ```hcl
 resource "databricks_grants" "sandbox" {
+  metastore    = "metastore_id"
   grant {
     principal  = "Data Engineers"
     privileges = ["CREATE_CATALOG", "CREATE_EXTERNAL_LOCATION"]