alternator: fix JSON in DescribeTable response

The structure's name in DescribeTable's output is supposed to be called
"Table", not "TableDescription". Putting in the wrong place caused the
driver's table creation waiters to fail.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

alternator/executor.cc

@@ -147,7 +147,7 @@ future<json::json_return_type> executor::describe_table(sstring content) {
     // maybe for TableId or TableArn use  schema.id().to_sstring().c_str();
     // Of course, the whole schema is missing!
     Json::Value response(Json::objectValue);
-    response["TableDescription"] = std::move(table_description);
+    response["Table"] = std::move(table_description);
     elogger.trace("returning {}", response.toStyledString());
     return make_ready_future<json::json_return_type>(make_jsonable(std::move(response)));
 }