Fix a broken design docs link about unused substs bug
Fix a broken design docs link about unused substs bug

https://github.com/rust-lang/project-const-generics/pull/21 removed the
link. This replaces it with an issue link mentioned on the design docs.

Signed-off-by: Yuki Okushi <0a792b09e3c7d426e6dc7b64bc4e95f557d97c14@2k36.org>

rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide