fix link

Author: steveklabnik

_posts/2017-02-02-Rust-1.15.md

@@ -253,10 +253,10 @@ check what your packages are doing and update to use `std::env`!
 
 [cargo/3368]:  https://github.com/rust-lang/cargo/pull/3368
 
-The `cargo test` command has [gained support for a `--all` flag][cargo/3321],
+The `cargo test` command has [gained support for a `--all` flag][cargo/3221],
 which is useful when you have a workspace.
 
-[cargo/3321]: https://github.com/rust-lang/cargo/pull/3321
+[cargo/3221]: https://github.com/rust-lang/cargo/pull/3221
 
 We now [Compile statically against the MSVC CRT][cargo/3363] on Windows, and
 [Link OpenSSL statically][cargo/3311] on Mac OS X.