fix link

Author: steveklabnik

_posts/2017-10-12-Rust-1.21.md

@@ -105,7 +105,7 @@ first steps toward having them work on stable Rust. Please check out the
 preview, and you'll hear more about these plans in the future.
 
 Finally, a few documentation improvements. First up, if you visit [the docs
-for `std::os`] (https://doc.rust-lang.org/stable/std/os/), which contains
+for `std::os`](https://doc.rust-lang.org/stable/std/os/), which contains
 operating-system specific functionality, you'll now see more than just `linux`,
 the platform we build the documentation on. We've long regretted that the hosted
 version of the documentation has been Linux-specific; this is a first step towards