Fixed incorrect regex in re_path() example.

docs/topics/http/urls.txt

@@ -201,7 +201,7 @@ Here's the example URLconf from earlier, rewritten using regular expressions::
         path('articles/2003/', views.special_case_2003),
         re_path(r'^articles/(?P<year>[0-9]{4})/$', views.year_archive),
         re_path(r'^articles/(?P<year>[0-9]{4})/(?P<month>[0-9]{2})/$', views.month_archive),
-        re_path(r'^articles/(?P<year>[0-9]{4})/(?P<month>[0-9]{2})/(?P<slug>[\w-_]+)/$', views.article_detail),
+        re_path(r'^articles/(?P<year>[0-9]{4})/(?P<month>[0-9]{2})/(?P<slug>[\w-]+)/$', views.article_detail),
     ]
 
 This accomplishes roughly the same thing as the previous example, except: