ax(ueps): replace placeholder name with concrete value in errTLVValueTooLarge comment

AX Principle 2: comments must show HOW with real values, not placeholder names.
`oversized` is a non-executable placeholder; `bytes.Repeat([]byte("x"), 256)` is
the concrete call that produces the error, matching the example already used in
the writeTLV body comment.

Co-Authored-By: Charon <charon@lethean.io>

pkg/ueps/packet.go

@@ -8,7 +8,7 @@ import (
 	"io"
 )
 
-// writeTLV(buffer, TagPayload, oversized) → errTLVValueTooLarge
+// writeTLV(buffer, TagPayload, bytes.Repeat([]byte("x"), 256)) → errTLVValueTooLarge
 // if err == errTLVValueTooLarge { /* value exceeded 255-byte TLV length limit */ }
 var errTLVValueTooLarge = sentinelError("TLV value too large for 1-byte length header")