Outdated download link to LLVM/Clang binary releases page

[cher-nov]

rust-bindgen/book/src/requirements.md

Lines 21 to 22 in b7b501f

	Alternatively, you can download and install the official pre-built binary from
	[LLVM download page](http://releases.llvm.org/download.html).

This link seems no longer updated, as it was largely superseded with https://releases.llvm.org and their GitHub Releases page.