Improve proposed pattern for Redirector extension

The old pattern matches all host names that end with medium.com. The new
pattern matches only medium.com and its sub-domains. For example, the
old pattern would have matched
https://foomedium.com/@user/post-123456abcdef.

src/pages/home/index_page.cr

@@ -53,7 +53,7 @@ class Home::IndexPage < MainLayout
           end
           li do
             strong "Include pattern: "
-            code ".*medium.com/(.*)"
+            code "^https?://(?:.*\\.)*medium.com/(.*)$"
           end
           li do
             strong "Redirect to: "