doc: use the more convenient 'cargo run' command

This lovely command has already been introduced, so let's take advantage
of it.

Author: tshepang

src/doc/guide.md

@@ -2682,9 +2682,9 @@ $ cargo new modules --bin
 Let's double check our work by compiling:
 
 ```{bash,ignore}
-$ cargo build
+$ cargo run
    Compiling modules v0.0.1 (file:///home/you/projects/modules)
-$ ./target/modules
+     Running `target/modules`
 Hello, world!
 ```