docs: amdgpu: fix a warning when building the documentation

As reported by Sphinx:

	Documentation/gpu/amdgpu.rst:200: WARNING: Inline emphasis start-string without end-string.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Documentation/gpu/amdgpu.rst

@@ -206,8 +206,8 @@ pp_power_profile_mode
 .. kernel-doc:: drivers/gpu/drm/amd/pm/amdgpu_pm.c
    :doc: pp_power_profile_mode
 
-*_busy_percent
-~~~~~~~~~~~~~~
+\*_busy_percent
+~~~~~~~~~~~~~~~
 
 .. kernel-doc:: drivers/gpu/drm/amd/pm/amdgpu_pm.c
    :doc: gpu_busy_percent