docs: update CONTRIBUTING

Author: lucaslarroche

.github/CONTRIBUTING.md

@@ -17,6 +17,6 @@ We recommend that you first [suggest your feature idea in our discussion forums]
 
 [`dev`](https://github.com/picocss/pico/tree/dev) branch is open for pull requests on `v2`.
 
-[`v1-dev`](https://github.com/picocss/pico/tree/dev) branch is open to pull requests on `v1`.
+[`v1-dev`](https://github.com/picocss/pico/tree/v1-dev) branch is open to pull requests on `v1`.
 
 **Do not edit [`/css`](https://github.com/picocss/pico/tree/master/css) files directly.** Edit the source files in [`/scss`](https://github.com/picocss/pico/tree/master/scss), then recompile the [`/css`](https://github.com/picocss/pico/tree/master/css) files with `yarn build`.