The new path() syntax in Django 2.0 does not use regular expressions. You want something like:
path('<int:album_id>/', views.detail, name="detail"),
If you want to use a regular expression, you can use re_path().
re_path(r'^(?P<album_id>[0-9])/$', views.detail, name="detail"),
The old url() still works and is now an alias to re_path, but it is likely to be deprecated in future.
url(r'^(?P<album_id>[0-9])/$', views.detail, name="detail"),