Remove duplicated slash in forum url in README

Author: marijnh

README.md

@@ -2,7 +2,7 @@
 
 # @codemirror/legacy-modes [![NPM version](https://img.shields.io/npm/v/@codemirror/legacy-modes.svg)](https://www.npmjs.org/package/@codemirror/legacy-modes)
 
-[ [**WEBSITE**](https://codemirror.net/) | [**ISSUES**](https://github.com/codemirror/dev/issues) | [**FORUM**](https://discuss.codemirror.net//) | [**CHANGELOG**](https://github.com/codemirror/legacy-modes/blob/main/CHANGELOG.md) ]
+[ [**WEBSITE**](https://codemirror.net/) | [**ISSUES**](https://github.com/codemirror/dev/issues) | [**FORUM**](https://discuss.codemirror.net/) | [**CHANGELOG**](https://github.com/codemirror/legacy-modes/blob/main/CHANGELOG.md) ]
 
 This package implements a collection of ported [stream
 language](https://codemirror.net/docs/ref#language.StreamParser) modes for