Avoid introducing run twice

As it stands, getting-started and guessing-game both introduce `run` as
a new command. The second should probably make it clear that the reader
has seen it before :)

Author: BlueSpaceCanary

src/doc/book/getting-started.md

@@ -505,6 +505,9 @@ $ cargo run
 Hello, world!
 ```
 
+The `run` command comes in handy when you need to rapidly iterate on a
+project.
+
 Notice that this example didn’t re-build the project. Cargo figured out that
 the file hasn’t changed, and so it just ran the binary. If you'd modified your
 source code, Cargo would have rebuilt the project before running it, and you

src/doc/book/guessing-game.md

@@ -56,9 +56,7 @@ $ cargo build
 Excellent! Open up your `src/main.rs` again. We’ll be writing all of
 our code in this file.
 
-Before we move on, let me show you one more Cargo command: `run`. `cargo run`
-is kind of like `cargo build`, but it also then runs the produced executable.
-Try it out:
+Remember the `run` command from last chapter? Try it out again here:
 
 ```bash
 $ cargo run
@@ -67,9 +65,8 @@ $ cargo run
 Hello, world!
 ```
 
-Great! The `run` command comes in handy when you need to rapidly iterate on a
-project. Our game is such a project, we need to quickly test each
-iteration before moving on to the next one.
+Great! Our game is just the kind of project `run` is good for: we need
+to quickly test each iteration before moving on to the next one.
 
 # Processing a Guess