sdlwiki: SDL3 advises users to include through 'SDL3/SDL.h'

.wikiheaders-options

@@ -4,7 +4,7 @@ incsubdir = include/SDL3
 wikisubdir =
 readmesubdir = docs
 apiprefixregex = (SDL_|SDLK_|KMOD_|AUDIO_)
-mainincludefname = SDL.h
+mainincludefname = SDL3/SDL.h
 versionfname = include/SDL3/SDL_version.h
 versionmajorregex = \A\#define\s+SDL_MAJOR_VERSION\s+(\d+)\Z
 versionminorregex = \A\#define\s+SDL_MINOR_VERSION\s+(\d+)\Z