Update suggested.md

I imagine that the book meant that a full build isn't needed since it gave as an example the addition of a debug! statement

src/building/suggested.md

@@ -155,7 +155,7 @@ lets you use `cargo fmt`.
 Sometimes just checking
 whether the compiler builds is not enough. A common example is that
 you need to add a `debug!` statement to inspect the value of some
-state or better understand the problem. In that case, you really need
+state or better understand the problem. In that case, you don't really need
 a full build. By bypassing bootstrap's cache invalidation, you can often get
 these builds to complete very fast (e.g., around 30 seconds). The only
 catch is this requires a bit of fudging and may produce compilers that