Documentation/gpu: correct path of reference

The path to GPU documentation is Documentation/gpu
rather than Documentation/GPU

This appears to have been introduced by commit ba162ae7
("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>