The path to GPU documentation is Documentation/gpu
rather than Documentation/GPU
This appears to have been introduced by commit ba162ae749
("Documentation/gpu: Introduce a simple contribution list for display code")
Flagged by make htmldocs.
Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>