`cargo doc` warnings

[tmfink]

Building docs with cargo doc causes several warnings:

$ cargo doc
...
warning: unresolved link to `0`
  --> capstone-rs/src/arch/x86.rs:89:16
   |
89 |     /// prefix[0] indicates REP/REPNE/LOCK prefix (See `X86_PREFIX_REP`/`REPNE`/`LOCK`)
   |                ^ no item named `0` in scope
   |
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
...