docs: fixup link in radv docs

This is not how to add links, this is how to use the default-role. Let's
correct it.

Fixes: 3ba3e00750 ("radv: Add some documentation.")
Reviewed-by: Dylan Baker <None>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32831>

docs/drivers/radv.rst

@@ -176,7 +176,7 @@ Registers in shaders (not to be confused with registers in the CP):
 * SGPR - scalar general purpose register: same value within a wave
 
 For further reading, AMD has publised whitepapers and documentation for the GCN and RDNA GPU architectures.
-`These can be found on their GPUOpen site. <https://gpuopen.com/amd-gpu-architecture-programming-documentation/>`
+`These can be found on their GPUOpen site. <https://gpuopen.com/amd-gpu-architecture-programming-documentation/>`__
 
 Debugging
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