OS X -> macOS (#1934)

Fix all documentation references of "OS X" to be "macOS", Apple's new brand.

Code is left unchanged because we're matching .NET Core.

docs/building/internals.md

@@ -1,7 +1,7 @@
 Internals of build process
 =========================================
 
-The purpose of this document is to explain build process **internals** with subtle nuances. 
+The purpose of this document is to explain build process **internals** with subtle nuances.
 This document is not by any means complete.
 The ultimate source of truth is the code in `.\build.psm1` that's getting executed on the corresponding CI system.
 
@@ -14,7 +14,7 @@ Top directory
 We are calling `dotnet` tool build for `$Top` directory
 
 - `src\powershell-win-core` for CoreCLR on Windows.
-- `src\powershell-unix` for CoreCLR on Linux and OS X.
+- `src\powershell-unix` for CoreCLR on Linux and macOS.
 - `src\powershell-win-full` for FullCLR builds (Windows only)
 
 ### Dummy dependencies

docs/building/macos.md

@@ -1,10 +1,10 @@
-Build PowerShell on OS X
+Build PowerShell on macOS
 ========================
 
 This guide supplements the [Linux instructions](./linux.md), as
-building on OS X is almost identical.
+building on macOS is almost identical.
 
-.NET Core (and by transitivity, us) only supports OS X 10.11, per
+.NET Core (and by transitivity, us) only supports macOS 10.11, per
 CoreFX issue #[7731][].
 
 [7731]: https://github.com/dotnet/corefx/issues/7731
@@ -12,9 +12,9 @@ CoreFX issue #[7731][].
 Environment
 ===========
 
-You will want [Homebrew](http://brew.sh/), the missing package manager for OS X.
+You will want [Homebrew](http://brew.sh/), the missing package manager for macOS.
 Once installed, follow the same instructions to download and
-install a self-hosted copy of PowerShell on your OS X machine,
+install a self-hosted copy of PowerShell on your macOS machine,
 and use`Start-PSBootstrap` to install the dependencies.
 
 The `Start-PSBootstrap` function does the following: