post: v6.18: fix report link

It is at the root dir, not the current dir with the date.

Reported-by: Tim Hummel
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

Author: matttbe

_posts/2025-10-16-v6.18-with-many-features.md

@@ -137,7 +137,7 @@ work! No security issues have been found 🎉
 The report mentioned one attention point: the plugin directory should not be
 world writeable, not to let other pieces of code executed with extra permissions
 (`CAP_NET_ADMIN`). The full report is available
-[here](assets/202503-mptcpd-security-report.pdf).
+[here](/assets/202503-mptcpd-security-report.pdf).
 
 In terms of improvements, it is good to note that mptcpd is now available in
 more Linux distributions: OpenWrt, Alpine Linux, NixOS, etc. A future v0.14