link to raw string docs in libregex docs

Fixes #17023

src/libregex/lib.rs

@@ -41,7 +41,8 @@
 //! it to match anywhere in the text. Anchors can be used to ensure that the
 //! full text matches an expression.
 //!
-//! This example also demonstrates the utility of raw strings in Rust, which
+//! This example also demonstrates the utility of [raw
+//! strings](../reference.html#character-and-string-literals) in Rust, which
 //! are just like regular strings except they are prefixed with an `r` and do
 //! not process any escape sequences. For example, `"\\d"` is the same
 //! expression as `r"\d"`.