ARROW-2043: [C++] change description from OS X to macOS

Author: yosuke shiro <ys2014hs@gmail.com>

Closes #1521 from shiro615/change-description-from-osx-to-macos and squashes the following commits:

ab03f72 [yosuke shiro] [C++] change description from OS X to macOS

Author: shiro615

cpp/README.md

@@ -39,7 +39,7 @@ sudo apt-get install cmake \
      libboost-system-dev
 ```
 
-On OS X, you can use [Homebrew][1]:
+On macOS, you can use [Homebrew][1]:
 
 ```shell
 git clone https://github.com/apache/arrow.git