default tech notes has bad path for cookie graphic

[davewhiteland]

Needs leading slash on /assets/img/cookie.png?

https://github.com/buggyrace/buggy-race-server/blob/main/tech_notes/content/tech-notes/secret-key.md?plain=1#L77-L79

https://demo.buggyrace.net/tech-notes/secret-key#login-example

[Not editing any tech notes while RHUL CS1998 teaching term is still running, to avoid conflicts with the custom tech notes. (Maybe github modules handle this better, but there is a strong case for doing custom tech notes in a different dir, and changing the config settings _TECH_NOTES_* which already exist... maybe expose those as editable config precisely for this scenario.

The exciting cookie should look like this :
https://demo.buggyrace.net/assets/img/cookie.png