Skip string allocation in benchmark/time_at.yml

and also drop a weird newline from benchmark/array_sample.yml.

benchmark/array_sample.yml

@@ -1,4 +1,3 @@
-
 prelude: ary = (1..10_000).to_a
 benchmark:
   - ary.sample

benchmark/time_at.yml

@@ -1,3 +1,5 @@
+prelude: |
+  # frozen_string_literal: true
 benchmark:
   - 'Time.at(0)'
   - 'Time.at(0, 500)'