Fix docs/examples/multi-uv.c deprecated symbol

[rkmathi]

I found one of libuv's symbol is outdated in docs/examples/multi-uv.

When I compile this example, I get a warning such as:

$ gcc -I../../include -I../../out/include/curl -I../../../../libuv/libuv/out/include multi-uv.c
multi-uv.c: In function ‘start_timeout’:        
multi-uv.c:168:30: warning: passing argument 2 of ‘uv_timer_start’ from incompatible pointer type [-Wincompatible-pointer-types]
     uv_timer_start(&timeout, on_timeout, timeout_ms, 0);           
                              ^~~~~~~~~~                                   
In file included from multi-uv.c:37:0:                              
../../../../libuv/libuv/out/include/uv.h:785:15: note: expected ‘uv_timer_cb {aka void (*)(struct uv_timer_s *)}’ but argument is of type ‘void (*)(uv_timer_t *, int) {aka void (*)(struct uv_timer_s *, int)}’
 UV_EXTERN int uv_timer_start(uv_timer_t* handle,

I checked libuv/libuv repository and found symbol void (*uv_timer_cb)(uv_timer_t* handle, int status); is outdated.
(diff: libuv/libuv@db2a907#diff-56343bd7ad8bf449c2693082fb3e4081L408 )

[coveralls]

Coverage decreased (-0.04%) to 73.217% when pulling 520492c on rkmathi:fix-uv-example into 6c59074 on curl:master.

[bagder]

Thanks!