Remove unnecessary pub from example code (#657)

Author: Marcono1234

book/src/getting_started.md

@@ -42,7 +42,7 @@ below for an explanation of this code):
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 use mycrate::fibonacci;
 
-pub fn criterion_benchmark(c: &mut Criterion) {
+fn criterion_benchmark(c: &mut Criterion) {
     c.bench_function("fib 20", |b| b.iter(|| fibonacci(black_box(20))));
 }