Fix reference to 'librlibc' in libcore docs

Fixes #23059

Author: wesleywiser

src/libcore/lib.rs

@@ -35,8 +35,7 @@
 //!   often generated by LLVM. Additionally, this library can make explicit
 //!   calls to these functions. Their signatures are the same as found in C.
 //!   These functions are often provided by the system libc, but can also be
-//!   provided by `librlibc` which is distributed with the standard rust
-//!   distribution.
+//!   provided by the [rlibc crate](https://crates.io/crates/rlibc).
 //!
 //! * `rust_begin_unwind` - This function takes three arguments, a
 //!   `fmt::Arguments`, a `&str`, and a `usize`. These three arguments dictate