Broken link in SSE extension docs

[gwk]

https://htmx.org/extensions/sse/ links to https://htmx.org/extensions/sse/@web-sockets.md which currently 404s.

[Telroshan]

Good catch! If you are interested, feel free to directly submit PRs to fix documentation, we don't require prior approved issues for this matter (we only do for changes to the lib's code itself)