Make book ordering more natural

Vectors come up in sections dedicated to ownership with the assumption that we know something about it but we haven't seen it yet. On the other hand, you need not know anything about ownership or lifetimes to understand the basics of vectors covered in the vector section of the book. Additionally, by moving it where it is there is a natural progression from loops to an iterative type which discusses for loops. This kind of interaction is generally better for learning.

I would like to have moved the struct section as well but I'm less confident about how to handle it since the ownership sections discuss structs and the structs section talks about mutable borrow.

src/doc/book/SUMMARY.md

@@ -9,6 +9,7 @@
     * [Comments](comments.md)
     * [if](if.md)
     * [Loops](loops.md)
+    * [Vectors](vectors.md)
     * [Ownership](ownership.md)
     * [References and Borrowing](references-and-borrowing.md)
     * [Lifetimes](lifetimes.md)
@@ -18,7 +19,6 @@
     * [Match](match.md)
     * [Patterns](patterns.md)
     * [Method Syntax](method-syntax.md)
-    * [Vectors](vectors.md)
     * [Strings](strings.md)
     * [Generics](generics.md)
     * [Traits](traits.md)