remove double on

Author: steveklabnik

posts/2018-12-20-Rust-1.31.1.md

@@ -23,7 +23,7 @@ appropriate page on our website, and check out the [detailed release notes for
 
 ## What's in 1.31.1 stable
 
-This patch release fixes a build failure on on `powerpc-unknown-netbsd` by
+This patch release fixes a build failure on `powerpc-unknown-netbsd` by
 way of [an update to the `libc`
 crate](https://github.com/rust-lang/rust/pull/56562) used by the compiler.