metainfo.yaml: Fix macOS entry
Fixes the following cmake warning: "CMake Warning at /usr/share/ECM/kde-modules/KDEMetaInfoPlatformCheck.cmake:69 (message): Found platform not recognized by the metainfo platform parser: MacOSX"
Fixes the following cmake warning: "CMake Warning at /usr/share/ECM/kde-modules/KDEMetaInfoPlatformCheck.cmake:69 (message): Found platform not recognized by the metainfo platform parser: MacOSX"