fix: check correct nibble values for alphabetic nibbles

src/main.rs

@@ -346,7 +346,7 @@ fn main() -> Result<()> {
         let last = prefix.as_bytes()[prefix.len() - 1];
         let idx = match last {
             48..=57 => last - 48,
-            97..=102 => last - 97,
+            97..=102 => last - 97 + 10,
             _ => unreachable!(),
         };
         let nibble = NIBBLES[idx as usize];