Fixed CMake argument omission causing build failure in Shader modules chapter

[butchhartman]

The CMake documentation defines the version of cmake_parse_arguments() used in the add_slang_shader_target() function provided in this chapter with the following signature:

cmake_parse_arguments(<prefix> <options> <one_value_keywords>
                      <multi_value_keywords> <args>...)

The current usage of cmake_parse_arguments() in this chapter produces a multiple definition warning and causes build failures due to ${ARGN} being interpreted as the <multi_value_keywords> parameter.

The proposed solution is to add an additional empty set of quotation marks before the "SOURCES" argument to properly pass the arguments to the ${SHADER_SOURCES} variable.

This categorizes ${SHADER_SOURCES} as a multi value keyword. I do not know if ${SHADER_SOURCES} was intended to be a multi or single value keyword, but I opted to select multi value keyword to be consistent with the plurality used throughout the chapter.

This was tested using CMake version 3.29