Fix 404 error when attempting to download OpenVSX extensions (#762)

This change fixes this error when running `(dap-node-setup)`:
`url-insert-file-contents: https://open-vsx.org/api/ms-vscode/node-debug2/latest/: Not found`

The problem was that the OpenVSX URL template string had a trailing
slash, which is not handled by OpenVSX's Registry API Controller:
https://github.com/eclipse/openvsx/blob/master/server/src/main/java/org/eclipse/openvsx/RegistryAPI.java#L348

I can't really find a blame in the OpenVSX repo which could've caused
the extraneous trailing slash. Maybe the update to Java 17?

As you can see, there's no trailing slash at the end, so the route
doesn't get mached in the controller and causes a 404 Not Found.

Just removing the slash does the job and `(dap-node-setup)` runs successfully.

References:
- #554 (comment)
- #700

dap-utils.el

@@ -61,7 +61,7 @@
   :type 'string)
 
 (defcustom dap-utils-openvsx-extension-api-url
-  "https://open-vsx.org/api/%s/%s/%s/"
+  "https://open-vsx.org/api/%s/%s/%s"
   "Open VSX extension api url."
   :group 'dap-utils
   :type 'string)