Replaces @tag with @describetag in sns_test.exs

[darrenklein]

When running tests, I noticed the following warning related to sns_test.exs:

$ mix test test/lib/sns_test.exs
Running ExUnit with seed: 795201, max_cases: 20

warning: found unused @tag before "describe", did you mean to use @describetag?
  (ex_unit 1.19.5) lib/ex_unit/callbacks.ex:802: ExUnit.Callbacks.__describe__/4
  test/lib/sns_test.exs:537: (module)
  (elixir 1.19.5) src/elixir_compiler.erl:81: :elixir_compiler.dispatch/4
  (elixir 1.19.5) src/elixir_compiler.erl:56: :elixir_compiler.compile/4
  (elixir 1.19.5) src/elixir_module.erl:476: :elixir_module.eval_form/7
  (elixir 1.19.5) src/elixir_module.erl:163: :elixir_module.compile/7
  (elixir 1.19.5) src/elixir_compiler.erl:42: :elixir_compiler.maybe_fast_compile/2
  (elixir 1.19.5) src/elixir_lexical.erl:17: :elixir_lexical.run/3
  (elixir 1.19.5) src/elixir_compiler.erl:21: :elixir_compiler.quoted/3
  (elixir 1.19.5) lib/kernel/parallel_compiler.ex:648: Kernel.ParallelCompiler.require_file/2
  (elixir 1.19.5) lib/kernel/parallel_compiler.ex:531: anonymous fn/5 in Kernel.ParallelCompiler.spawn_workers/8

*......................................

I believe that the intent was to skip all of the tests in the describe block, so the fix was to add a @describetag declaration inside the block.