DOCS(dev): Fix UTP to TCP ref

The cross-document reference apparently worked on readthedocs (our previous docs), but does not work on GitHub.

The cross-document figure reference is replaced with a file reference.

docs/dev/network-protocol/voice_data.rst

@@ -321,7 +321,7 @@ Tunneling audio over TCP
 
 If the UDP channel isn't available the voice packets can be transmitted through
 the TCP transport used for the control channel. These messages use the normal
-TCP prefixing, as shown in figure :ref:`mumble-packet`: 16-bit message type
+TCP prefixing, as shown in `Protocol Stack TCP <protocol_stack_tcp.rst>`_: 16-bit message type
 followed by 32-bit message length. However unlike other TCP messages, the audio
 packets are not encoded as protocol buffer messages but instead the raw audio
 packet described in `Packet format`_ should be written to the TCP socket