remove old reference to "nightly" feature

[Doug A]

Hi, found this but it looks like this is outdated and can be removed?

wasm-bindgen/guide/src/reference/passing-rust-closures-to-js.md

Lines 66 to 71 in 12889ef

 The `Closure` type is currently behind a feature which needs to be enabled: 
  
 ```toml 
 [dependencies] 
 wasm-bindgen = {version = "^0.2", features = ["nightly"]} 
 ``` 

[daxpedda]

Yes, would you mind making a PR?

[Doug A]

Yep, done!