Fix tester link

Author: ComputerDruid

README.md

@@ -18,7 +18,7 @@ compiletest_rs = "*"
 ```
 
 By default, `compiletest-rs` should be able to run on both stable, beta and
-nightly channels of rust. We use the [`tester`](tester) fork of Rust's builtin
+nightly channels of rust. We use the [`tester`][tester] fork of Rust's builtin
 `test` crate, so that we don't have require nightly. If you are running nightly
 and want to use Rust's `test` crate directly, you need to have the rustc development
 libraries install (which you can get by running `rustup component add rustc-dev