Fix regression in ComputerSystem

Despite the pseudo non-plural name "Links.Chassis", the Redfish schema
states that Chassis is an array of chassis, not a singular chassis.
While odd, we should obey the specification.

This resolves the regression introduced by 1476687de

Tested: Code compiles, code inspection looks correct.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I4d6d0554865e412137c44562d173a6efe3f88f4a
diff --git a/redfish-core/lib/systems.hpp b/redfish-core/lib/systems.hpp
index e136aea..58bab2d 100644
--- a/redfish-core/lib/systems.hpp
+++ b/redfish-core/lib/systems.hpp
@@ -2965,8 +2965,11 @@
             getMainChassisId(
                 asyncResp, [](const std::string& chassisId,
                               const std::shared_ptr<bmcweb::AsyncResp>& aRsp) {
-                    aRsp->res.jsonValue["Links"]["Chassis"]["@odata.id"] =
-                        "/redfish/v1/Chassis/" + chassisId;
+                    nlohmann::json::array_t chassisArray;
+                    nlohmann::json& chassis = chassisArray.emplace_back();
+                    chassis["@odata.id"] = "/redfish/v1/Chassis/" + chassisId;
+                    aRsp->res.jsonValue["Links"]["Chassis"] =
+                        std::move(chassisArray);
                 });
 
             getLocationIndicatorActive(asyncResp);