Auto merge of #30091 - vyp:doc-typo, r=bluss

Author: bors

src/doc/book/ffi.md

@@ -517,8 +517,6 @@ The [`libc` crate on crates.io][libc] includes type aliases and function
 definitions for the C standard library in the `libc` module, and Rust links
 against `libc` and `libm` by default.
 
-[libc]: https://crates.io/crates/libc
-
 # The "nullable pointer optimization"
 
 Certain types are defined to not be `null`. This includes references (`&T`,