Fix MulticastSocket example code

The example code works for all-ASCII strings, but DatagramPacket's
length argument takes the byte length and String.length() returns the
number of UTF-16 characters, so if a string has any non-ASCII
characters s.length() != s.getBytes().length.

Bug: 127912421
Test: make docs
Change-Id: I3e16780edf030807f9158ec76760262a0460b84c