[socket.dgram.op] replace "\effects Returns" with "\returns"

Do this for the returns clause for async_receive_from.

Author: LarsGullik

src/sockets.tex

@@ -2603,7 +2603,7 @@
 
 \begin{itemdescr}
 \pnum
-\effects Returns \tcode{async_receive_from(buffers, sender, socket_base::message_flags(), for\-ward<CompletionToken>(token))}.
+\returns \tcode{async_receive_from(buffers, sender, socket_base::message_flags(), forward<Com\-pletionToken>(token))}.
 \end{itemdescr}
 
 \begin{itemdecl}