fix relative link (#303)

Author: RalfJung

src/infra/toolstate.md

@@ -79,4 +79,4 @@ To add a new tool to be tracked, the following steps must be taken:
 [Reference]: https://doc.rust-lang.org/reference/
 [toolstate]: https://rust-lang-nursery.github.io/rust-toolstate/
 [toolstate repository]: https://github.com/rust-lang-nursery/rust-toolstate/
-[forge]: index.html
+[forge]: ../index.html