Book: ch2: cargo build has output even without changes

[nathany]

If you immediately run cargo build again without making any changes, you won’t get any output.

Not entirely true:

❯ cargo build
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs

[carols10cents]

Ah, true :) We just sent chapter 2 back to our editors, but I think we get one more chance at edits before print, sooo... good timing!

[TisonKun]

Note that there is no compiling.

[carols10cents]

Reopening this since this issue is about chapter 2 and PR #1044 was for chapter 1.