message: Fix 'an streaming' typo
diff: @@ -36,7 +36,7 @@ namespace tink {
  // the ciphertext prefix.
  class StreamingAeadWrapper : public PrimitiveWrapper<StreamingAead> {
  public:
  - // Returns an StreamingAead-primitive that uses StreamingAead-instances
  + // Returns a StreamingAead-primitive that uses StreamingAead-instances
  // provided in 'streaming_aead_set', which must be non-NULL and must contain
  // a primary instance.
  util::StatusOr<std::unique_ptr<StreamingAead>> Wrap(
  