arm64: Fix reference to docs for ARM64_TAGGED_ADDR_ABI

The referenced file does not exist, but tagged-address-abi.rst does.
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/Kconfig

@@ -1135,7 +1135,7 @@ config ARM64_TAGGED_ADDR_ABI
 	  When this option is enabled, user applications can opt in to a
 	  relaxed ABI via prctl() allowing tagged addresses to be passed
 	  to system calls as pointer arguments. For details, see
-	  Documentation/arm64/tagged-address-abi.txt.
+	  Documentation/arm64/tagged-address-abi.rst.
 
 menuconfig COMPAT
 	bool "Kernel support for 32-bit EL0"