doc: fix code formatting

doc/languages-frameworks/rust.section.md

@@ -146,6 +146,7 @@ where they are known to differ. But there are ways to customize the argument:
        rustc.platform = { foo = ""; bar = ""; };
      };
    }
+   ```
    will result in:
    ```shell
    --target /nix/store/asdfasdfsadf-thumb-crazy.json # contains {"foo":"","bar":""}