Fix forum link in readme

Author: marijnh

README.md

@@ -1,6 +1,6 @@
 # codemirror [![NPM version](https://img.shields.io/npm/v/codemirror)](https://www.npmjs.org/package/codemirror)
 
-[ [**WEBSITE**](https://codemirror.net/) | [**DOCS**](https://codemirror.net/docs/ref/#codemirror) | [**ISSUES**](https://github.com/codemirror/dev/issues) | [**FORUM**](https://discuss.codemirror.net/c/next/) | [**CHANGELOG**](https://github.com/codemirror/basic-setup/blob/main/CHANGELOG.md) ]
+[ [**WEBSITE**](https://codemirror.net/) | [**DOCS**](https://codemirror.net/docs/ref/#codemirror) | [**ISSUES**](https://github.com/codemirror/dev/issues) | [**FORUM**](https://discuss.codemirror.net//) | [**CHANGELOG**](https://github.com/codemirror/basic-setup/blob/main/CHANGELOG.md) ]
 
 This package provides an example configuration for the
 [CodeMirror](https://codemirror.net/) code editor. The actual editor