Fix Sphinx warning about cmake code block

Lexer for CMake could not parse this broken grammar, because it is ".cmake.in", not just ".cmake".

CMake itself solves this problems by declaring such code blocks as generic "::" blocks instead. For example, take a look at the last code block at CMakePackageConfigHelpers help page:

• https://cmake.org/cmake/help/v3.18/module/CMakePackageConfigHelpers.html#example-generating-package-files
• https://github.com/Kitware/CMake/blob/0464298956a204578aa8421ca0b84c089a97e0aa/Modules/CMakePackageConfigHelpers.cmake#L208