Fix code block in changelog

CHANGELOG.md

@@ -5,7 +5,7 @@
 #### Fix `--stdin` regression in 1.19.0 ([#6894](https://github.com/prettier/prettier/pull/6894) by [@lydell](https://github.com/lydell))
 
 <!-- prettier-ignore -->
-```jsx
+```
 // Prettier stable
 $ echo "test" | prettier --stdin --parser babel
 [error] regeneratorRuntime is not defined