Documentation: Fix early-microcode.txt references after file rename

The file Documentation/x86/early-microcode.txt was renamed to
Documentation/x86/microcode.txt in 0e3258753f, but it was still
referenced by its old name in a three places:

* Documentation/x86/00-INDEX
* arch/x86/Kconfig
* arch/x86/kernel/cpu/microcode/amd.c

This commit updates these references accordingly.

Fixes: 0e3258753f ("x86/microcode: Document the three loading methods")
Signed-off-by: Jaak Ristioja <jaak@ristioja.ee>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

arch/x86/kernel/cpu/microcode/amd.c

@@ -58,7 +58,7 @@ static u8 amd_ucode_patch[PATCH_MAX_SIZE];
 
 /*
  * Microcode patch container file is prepended to the initrd in cpio
- * format. See Documentation/x86/early-microcode.txt
+ * format. See Documentation/x86/microcode.txt
  */
 static const char
 ucode_path[] __maybe_unused = "kernel/x86/microcode/AuthenticAMD.bin";