Fix compat notes link

Author: Mark-Simulacrum

posts/2019-12-19-Rust-1.40.0.md

@@ -240,7 +240,7 @@ In Rust 1.40.0 the following functions and macros were stabilized:
 
 [relnotes-cargo]: https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-140-2019-12-19
 [relnotes-clippy]: https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md#rust-140
-[compat-notes]: https://github.com/rust-lang/rust/blob/stable/RELEASES.md#compatibility-notes
+[compat-notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#compatibility-notes
 
 There are other changes in the Rust 1.40.0 release: check out what changed in [Rust][notes], [Cargo][relnotes-cargo], and [Clippy][relnotes-clippy].