A mistake in the document guide.md

https://github.com/rust-lang/rust/blame/master/src/doc/guide.md#L2282 this line of code can not compile, 

```
type `&'static str` does not implement any method in scope named `parse`
```

Should replace all `parse()` with `from_str()` 

```
let input_num =  from_str::<uint>("5");
```
