s/Direcive/Directive/

doc/packed_data.rdoc

@@ -577,7 +577,7 @@ for one byte in the input or output string.
     [0, 1, 2].pack("CCX2C")   # => "\x02"
     "\x00\x02".unpack("CCXC") # => [0, 2, 2]
 
-== Null Byte Direcive
+== Null Byte Directive
 
 - <tt>'x'</tt> - Null byte: