Fix mistake in trait.md

[xiaochuanyu]

Originally in an example it reads as follows:

fn inverse<T>() -> T
        // this is using ConvertTo as if it were "ConvertFrom<i32>"
         where i32: ConvertTo<T> {
     42.convert()
 }

There was no mention of ConvertFrom elsewhere in the page other than in this comment. Is this supposed to be ConvertTo<i64> ?
I'm confused by this example.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

@bors: r+ 865d6c3 rollup

Thanks!