Documentation: amd-pstate: Use internal link to kselftest

Convert kselftest docs link to internal cross-reference.

Signed-off-by: Bagas Sanjaya <[email protected]>

Author: bagasme

Documentation/admin-guide/pm/amd-pstate.rst

@@ -798,5 +798,4 @@ Reference
 .. [3] Processor Programming Reference (PPR) for AMD Family 19h Model 51h, Revision A1 Processors
        https://www.amd.com/system/files/TechDocs/56569-A1-PUB.zip
 
-.. [4] Linux Kernel Selftests,
-       https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html
+.. [4] Documentation/dev-tools/kselftest.rst