chore(readme): tweak security and code signing sections

Author: SapphoSys

README.md

@@ -206,11 +206,13 @@ Chiri is licensed under the [<span aria-hidden="true">&nearr;</span> zlib/libpng
 # Privacy
 See [<span aria-hidden="true">&nearr;</span> PRIVACY.md][repo-privacy] for details on what data Chiri processes and how.
 
-# Security
-Found a security issue? Please report it privately — see [<span aria-hidden="true">&nearr;</span> SECURITY.md][repo-security] for details.
+## Security
+Found a security issue? Please report it privately. See [<span aria-hidden="true">&nearr;</span> SECURITY.md][repo-security] for details.
 
-# Code signing
-Free code signing on Windows is provided by [<span aria-hidden="true">&nearr;</span> SignPath.io][signpath-io], certificate by the [<span aria-hidden="true">&nearr;</span> SignPath Foundation][signpath-foundation]. See [<span aria-hidden="true">&nearr;</span> CODE_SIGNING.md][repo-code-signing] for the full code signing policy.
+## Code signing
+Free code signing on Windows is provided by [<span aria-hidden="true">&nearr;</span> SignPath.io][signpath-io], certificate by the [<span aria-hidden="true">&nearr;</span> SignPath Foundation][signpath-foundation].
+
+See [<span aria-hidden="true">&nearr;</span> CODE_SIGNING.md][repo-code-signing] for the full code signing policy.
 
 [donate-kofi]: https://ko-fi.com/solelychloe
 [donate-liberapay]: https://liberapay.com/chloe