Cargo build now builds to /target/debug/, changed the docs to reflect that.

Deleted trailing space in hello-cargo.md

Added note about build --release in hello-cargo

thanks to steven klabnik for pointing it out for me

edited out redundant wording

src/doc/trpl/hello-cargo.md

@@ -78,16 +78,16 @@ Once you have this file in place, we should be ready to build! Try this:
 ```bash
 $ cargo build
    Compiling hello_world v0.0.1 (file:///home/yourname/projects/hello_world)
-$ ./target/hello_world
+$ ./target/debug/hello_world
 Hello, world!
 ```
 
 Bam! We build our project with `cargo build`, and run it with
-`./target/hello_world`. This hasn't bought us a whole lot over our simple use
+`./target/debug/hello_world`. This hasn't bought us a whole lot over our simple use
 of `rustc`, but think about the future: when our project has more than one
 file, we would need to call `rustc` more than once, and pass it a bunch of options to
 tell it to build everything together. With Cargo, as our project grows, we can
-just `cargo build` and it'll work the right way.
+just `cargo build` and it'll work the right way. When you're project is finally ready for release, you can use `cargo build --release` to compile your crates with optimizations.
 
 You'll also notice that Cargo has created a new file: `Cargo.lock`.
 
@@ -163,4 +163,4 @@ much more in-depth guide to Cargo can be found [here](http://doc.crates.io/guide
 
 Now that you've got the tools down, let's actually learn more about the Rust
 language itself. These are the basics that will serve you well through the rest
-of your time with Rust.
+of your time with Rust.