DOC: Update to the official name of Apple OS systems.

Update the name of Apple OS system to the current name: change the
appearances of OS X, and Mac OSX to macOS:
https://www.apple.com/macos/high-sierra/

Change-Id: Idd2bf1eb65d6a4c5d466f7723bd3c58482631776

Author: Jon Haitz Legarreta Gorroño

SoftwareGuide/Latex/Appendices/CodingStyleGuide.tex

@@ -205,7 +205,7 @@ \subsection{Portability}
 ITK is designed to build and is systematically tested on a set of target
 operating system/compiler combinations and results are reported continuously
 to the dashboards using \href{https://www.cdash.org/}{CDash}. These combinations
-include Linux, OS X, and Windows operating systems, and various versions of
+include Linux, macOS, and Windows operating systems, and various versions of
 compilers for each. This ensures that the code complies with the particular
 requirements on each of these environments. See Section \ref{sec:CDash}
 on page \pageref{sec:CDash} for further details.

SoftwareGuide/Latex/Introduction/Installation.tex

@@ -15,7 +15,7 @@ \chapter{Configuring and Building ITK}
 
 ITK has been developed and tested across different combinations of operating
 systems, compilers, and hardware platforms including Microsoft Windows, Linux on
-various architectures, Solaris/UNIX, Mac OSX, and Cygwin. Kitware is committed
+various architectures, Solaris/UNIX, macOS, and Cygwin. Kitware is committed
 to support the following compilers for building ITK:
 
 % List updated according to information provided here:
@@ -24,7 +24,7 @@ \chapter{Configuring and Building ITK}
 \begin{itemize}
 \item GCC 4.x
 \item Visual Studio 8 SP 1 (until 2015), 9 (until 2018), 10 (until 2020)
-\item Intel Compiler Suite 11.x, 12.x (including Mac OS X release)
+\item Intel Compiler Suite 11.x, 12.x (including macOS release)
 \item Darwin-c++-4.2 PPC (until 2015), x86\_64
 \item Win32-mingw-gcc-4.5
 \item Clang 3.3 and later
@@ -167,7 +167,7 @@ \subsection{Preparing CMake}
 \end{center}
 
 You can download binary versions for most of the popular platforms including
-Microsoft Windows, Mac OSX, Linux, PowerPC and IRIX. Alternatively you can
+Microsoft Windows, macOS, Linux, PowerPC and IRIX. Alternatively you can
 download the source code and build CMake on your system. Follow the instructions
 provided on the CMake web page for downloading and installing the software. The
 minimum version of CMake has been evolving along with the version of ITK. For