Update Rust version in the Rustonomicon Vec chapter

I verified that the final code compiles in Rust 1.9.0-nightly.

vec.md

@@ -2,7 +2,7 @@
 
 To bring everything together, we're going to write `std::Vec` from scratch.
 Because all the best tools for writing unsafe code are unstable, this
-project will only work on nightly (as of Rust 1.2.0). With the exception of the
+project will only work on nightly (as of Rust 1.9.0). With the exception of the
 allocator API, much of the unstable code we'll use is expected to be stabilized
 in a similar form as it is today.