move Deref coercions to syntax and semantics

It's a language feature, so it belongs here.

Author: steveklabnik

src/doc/trpl/SUMMARY.md

@@ -14,7 +14,6 @@
     * [Concurrency](concurrency.md)
     * [Error Handling](error-handling.md)
     * [FFI](ffi.md)
-    * [Deref coercions](deref-coercions.md)
 * [Syntax and Semantics](syntax-and-semantics.md)
     * [Variable Bindings](variable-bindings.md)
     * [Functions](functions.md)
@@ -53,6 +52,7 @@
     * [Casting between types](casting-between-types.md)
     * [Associated Types](associated-types.md)
     * [Unsized Types](unsized-types.md)
+    * [Deref coercions](deref-coercions.md)
     * [Macros](macros.md)
     * [`unsafe` Code](unsafe-code.md)
 * [Nightly Rust](nightly-rust.md)