--emit=ir is listed as valid choice in manual, but rustc does not accept it anymore

``` sh
$ man rustc | grep -- --emit | head -n1
       --emit=[asm,ir,bc,obj,link]
$ rustc src/lib.rs --crate-type=rlib --emit=ir
error: unknown emission type: `ir`
$ rustc --version
rustc 0.13.0-nightly (cc19e3380 2014-12-20 20:00:36 +0000)
```

The `ir` part in the manual should be changed to `llvm-ir`.
