Update error message

Author: siddhant-gupta-accenture

packages/react-router/__tests__/createRoutesFromChildren-test.tsx

@@ -216,6 +216,6 @@ describe("creating routes from JSX", () => {
           </Route>
         </Route>
       );
-    }).toThrow("An index route cannot have children routes.");
+    }).toThrow("An index route must not have child routes.");
   });
 });

packages/react-router/lib/components.tsx

@@ -585,7 +585,7 @@ export function createRoutesFromChildren(
     };
 
     if (element.props.children) {
-      invariant(!route.index, "An index route cannot have children routes.");
+      invariant(!route.index, "An index route must not have child routes.");
       route.children = createRoutesFromChildren(
         element.props.children,
         treePath