Fix alloc link in exotic-sizes for local docs

[cherryblossom000]

This changes https://doc.rust-lang.org/std/alloc/trait.GlobalAlloc.html#tymethod.alloc to ../std/alloc/trait.GlobalAlloc.html#tymethod.alloc so that it uses the local docs installation if the page is viewed with the local docs, instead of https://doc.rust-lang.org.