Fix error about required parameter (#14989)

See the docs: https://helmetjs.github.io/docs/ (maxAge is not a required parameter)

helmet/index.d.ts

@@ -81,7 +81,7 @@ declare namespace helmet {
     }
 
     export interface IHelmetHstsConfiguration {
-        maxAge: number;
+        maxAge?: number;
         includeSubdomains?: boolean;
         preload?: boolean;
         setIf?: IHelmetSetIfFunction,