How to access params to get a single object in $resource?

So I am trying to get a single "post" object from a list of posts but i don't know the name of passed params object name. Here's post-details component 'use strict';

angular.module('postDetail', ['ngRoute', 'postsService'])

.config(['$routeProvider', function($routeProvider) {
  $routeProvider.when('/posts/:postId', {
    templateUrl: 'post-detail/post-detail.template.html',
    controller: 'PostDetailController'
  });
}])

.controller('PostDetailController', ['$scope', '$http', 'postsService', '$routeParams', function($scope, $http, postsService, $routeParams) {
    $scope.post = postsService.getPost($routeParams.postId);
}]);

Here I am passing postId to the getPost method. BTW, here the value of postId is valid.

This is my Posts Service.

angular.
  module('postsService', ['ngResource']).
  factory('postsService', ['$resource',
    function($resource) {
      return $resource('https://jsonplaceholder.typicode.com/posts/:postId', {}, {
        getPost: {
          method: 'GET',
          params: {postId: '@postId'}
        }
      });
    }
  ]);

But here the "@postId" expression is empty and it tries to get the list of posts. How can i reference the passed parameter?

My post details template

<h3>{{post.title}}</h3>
<p>{{post.body}}</p>

Instead of:

$scope.post = postsService.getPost($routeParams.postId);

do:

$scope.post = postsService.getPost({
    postId: $routeParams.postId
});

Check out the example of how to use $resource class methods in the documentation.