Correct the entry point in librustc/README.md

[strega-nil]

No description provided.

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[strega-nil]

r? @huonw

[huonw]

@bors r+ rollup

[bors]

📌 Commit b2a38c3 has been approved by huonw

[lilyball]

Note: the README is woefully out of date. For example, it claims that all the driver code lives in librustc_trans. It also only documents 4 librustc* crates when there's now 13.