Fix `browse-url-default-scheme` custom :type

* lisp/net/browse-url.el (browse-url-default-scheme): Fix custom
:type.

lisp/net/browse-url.el

@@ -680,7 +680,9 @@ For example, when point is on an URL fragment like
 Note that if you set this to \"https\", websites that do not yet
 support HTTPS may not load correctly in your web browser.  Such
 websites are increasingly rare, but they do still exist."
-  :type 'string
+  :type '(choice (const :tag "HTTP" "http")
+                 (const :tag "HTTPS" "https")
+                 (string :tag "Something else" "https"))
   :version "29.1")
 
 (defun browse-url-url-at-point ()