rename the "flatten-recursive" benchmark to the more accurate "flatten-deep"

Author: Nicolas Laurent

bench/micro/array/flatten-recursive.rb renamed to bench/micro/array/flatten-deep.rb

@@ -14,6 +14,6 @@
   current_array = next_array
 end
 
-benchmark 'core-array-flatten-recursive' do
+benchmark 'core-array-flatten-deep' do
   array.flatten
 end