reference: "ffi" is normally in upper case

Author: tshepang

src/doc/reference.md

@@ -3931,7 +3931,7 @@ initialized; this is enforced by the compiler.
 The Rust compiler supports various methods to link crates together both
 statically and dynamically. This section will explore the various methods to
 link Rust crates together, and more information about native libraries can be
-found in the [ffi section of the book][ffi].
+found in the [FFI section of the book][ffi].
 
 In one session of compilation, the compiler can generate multiple artifacts
 through the usage of either command line flags or the `crate_type` attribute.