replace gitter with matrix room

missed in #276

codespan/README.md

@@ -3,15 +3,15 @@
 [![Continuous integration][actions-badge]][actions-url]
 [![Crates.io][crate-badge]][crate-url]
 [![Docs.rs][docs-badge]][docs-url]
-[![Gitter][gitter-badge]][gitter-lobby]
+[![Matrix][matrix-badge]][matrix-lobby]
 
 [actions-badge]: https://img.shields.io/github/workflow/status/brendanzab/codespan/Continuous%20integration
 [actions-url]: https://github.com/brendanzab/codespan/actions
 [crate-url]: https://crates.io/crates/codespan
 [crate-badge]: https://img.shields.io/crates/v/codespan.svg
 [docs-url]: https://docs.rs/codespan
 [docs-badge]: https://docs.rs/codespan/badge.svg
-[gitter-badge]: https://badges.gitter.im/codespan-rs/codespan.svg
-[gitter-lobby]: https://gitter.im/codespan-rs/Lobby
+[matrix-badge]: https://img.shields.io/badge/matrix-%23codespan%3Amatrix.org-blue.svg
+[matrix-lobby]: https://app.element.io/#/room/#codespan:matrix.org
 
 Utilities for dealing with source code locations.