Fix typo: "an storage" -> "a storage"

Author: jdufresne

activerecord/test/cases/encryption/performance/storage_performance_test.rb

@@ -57,7 +57,7 @@ def assert_storage_performance(size:, overload_less_than:, quiet: true)
 
       assert\
         overload_factor <= overload_less_than,
-        "Expecting an storage overload of #{overload_less_than} at most for #{size} bytes, but got #{overload_factor} instead"
+        "Expecting a storage overload of #{overload_less_than} at most for #{size} bytes, but got #{overload_factor} instead"
     end
 
     def encryptor