Fix typo in label max length

chapter2.md

@@ -57,7 +57,7 @@ We'll also need a function for writing query names in labeled form:
     fn write_qname(&mut self, qname: &str) -> Result<()> {
         for label in qname.split('.') {
             let len = label.len();
-            if len > 0x34 {
+            if len > 0x3f {
                 return Err("Single label exceeds 63 characters of length".into());
             }