Update addons.markdown

libuv link fix

PR-URL: nodejs/node-v0.x-archive#25885
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/addons.markdown

@@ -10,7 +10,7 @@ knowledge of several libraries:
    tree), which is also available
    [online](http://v8dox.com/).
 
- - [libuv](https://github.com/joyent/libuv), C event loop library.
+ - [libuv](https://github.com/libuv/libuv), C event loop library.
    Anytime one needs to wait for a file descriptor to become readable,
    wait for a timer, or wait for a signal to be received one will need
    to interface with libuv. That is, if you perform any I/O, libuv will