Move drop back in TRPL TOC

It's a trait implementation, so we need traits first.

Author: steveklabnik

src/doc/trpl/SUMMARY.md

@@ -33,11 +33,11 @@
     * [Structs](structs.md)
     * [Patterns](patterns.md)
     * [Method Syntax](method-syntax.md)
-    * [Drop](drop.md)
     * [Vectors](vectors.md)
     * [Strings](strings.md)
     * [Traits](traits.md)
     * [Operators and Overloading](operators-and-overloading.md)
+    * [Drop](drop.md)
     * [Generics](generics.md)
     * [if let](if-let.md)
     * [Trait Objects](trait-objects.md)