SUMMARY: update "Report a security bug" link

The kernel moved it from `admin-guide/` to `process/` in Linux commit
44ac5abac86b ("Documentation/security-bugs: move from admin-guide/
to process/").

Reported-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

SUMMARY.md

@@ -41,7 +41,7 @@
 
 ## Security
 
-  - [Report a security bug](https://docs.kernel.org/admin-guide/security-bugs.html)
+  - [Report a security bug](https://docs.kernel.org/process/security-bugs.html)
 
 ## Issue tracking