docs: Fixed DER encoding of DigestInfo for PIV signing command

DER encoding was missing the mandatory NULL value (05 00) to ensure signatures are compatible with OpenSSL/BoringSSL

Author: YourMJK

docs/users-manual/application-piv/commands.md

@@ -880,21 +880,22 @@ For example, if using PKCS 1 v 1.5, before calling, build the following block.
 
   For a 2048-bit key, the block is 256 bytes long (the leading 00 byte is one of the 256).
 
-  If the digest algorithm is SHA-256, the DER of the DigestInfo will be 49 bytes long:
+  If the digest algorithm is SHA-256, the DER of the DigestInfo will be 51 bytes long:
 
-  30 2f
-     30 0b
+  30 31
+     30 0d
         06 09
            60 86 48 01 65 03 04 02 01
+        05 00
      04 20
         <32-byte digest>
 
   The block to pass to the YubiKey will be
 
-  00 01 FF FF ... FF 00 \<49-byte DER of DigestInfo\>
+  00 01 FF FF ... FF 00 \<51-byte DER of DigestInfo\>
         ^          ^
         |          |
-        -------------- 204 bytes of 0xFF
+        -------------- 202 bytes of 0xFF
 ```
 
 PSS (Probabilistic Signature Scheme) is much more complicated. If you want to learn how to