Fix link to Contributors section in README

The dashes are also part of the link to this section in the GitHub
webview and without the link does not work on GitHub.
Although, the additional dashes could not fix the link for other editors
(e.g. VSCodium with "Markdwn All in One" or an JetBrains IDE).

README.md

@@ -53,7 +53,7 @@ Try Live Editor previews of future releases: <a href="https://develop.git.mermai
 - [Examples](#examples)
 - [Release](#release)
 - [Related projects](#related-projects)
-- [Contributors](#contributors)
+- [Contributors](#contributors---)
 - [Security and safe diagrams](#security-and-safe-diagrams)
 - [Reporting vulnerabilities](#reporting-vulnerabilities)
 - [Appreciation](#appreciation)