Update README.md

rust-lang#72827 changed it from `*.nix` to `Unix-like system`, but actually it should be `a Unix-like system`

Author: DutchGhost

README.md

@@ -23,7 +23,7 @@ or reading the [rustc dev guide][rustcguidebuild].
 
 [rustcguidebuild]: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html
 
-### Building on Unix-like system
+### Building on a Unix-like system
 1. Make sure you have installed the dependencies:
 
    * `g++` 5.1 or later or `clang++` 3.5 or later