Merge pull request #917 from wsh/ffi-fix

linkage.md: update link to FFI section of the Book.

src/linkage.md

@@ -8,7 +8,7 @@ statically and dynamically. This section will explore the various methods to
 link crates together, and more information about native libraries can be
 found in the [FFI section of the book][ffi].
 
-[ffi]: ../book/ffi.html
+[ffi]: ../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
 
 In one session of compilation, the compiler can generate multiple artifacts
 through the usage of either command line flags or the `crate_type` attribute.