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

* lisp/net/browse-url.el (browse-url-default-scheme): Fix custom
:type.
1 job for master in 2 minutes and 21 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Normal
failed #76100
test-lisp-net-inotify

00:02:21

 
Name Stage Failure
failed
test-lisp-net-inotify Normal
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WARNING: test-lisp-net-inotify-194a8f5c/junit-test-report.xml: no matching files. Ensure that the artifact path is relative to the working directory
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Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1