Fix a typo (#12)

I thought that benchmarkFib1 should be supposed to expose, how is it?

2-benchmarking/examples/fib/fib_test.go

@@ -20,7 +20,7 @@ func BenchmarkFib20(b *testing.B) {
 	}
 }
 
-func benchmarkFib1(b *testing.B) {
+func BenchmarkFib1(b *testing.B) {
 	for n := 0; n < b.N; n++ {
 		Fib(1)
 	}