message: Fixes reference to AES128_GCM
  There were a couple of places where `AeadKeyTemplates.AES_128_GCM` was mentioned. Replaced it with `AeadKeyTemplates.AES128_GCM` as it's the correct name of the constant.
diff: @@ -179,7 +179,7 @@ e.g., writing to a file:
  // Generate the key material...
  KeysetHandle keysetHandle = KeysetHandle.generateNew(
  - AeadKeyTemplates.AES_128_GCM);
  + AeadKeyTemplates.AES128_GCM);
  // and write it to a file.
  String keysetFilename = "my_keyset.json";
  @@ -204,7 +204,7 @@ as follows:
  // Generate the key material...
  KeysetHandle keysetHandle = KeysetHandle.generateNew(
  - AeadKeyTemplates.AES_128_GCM);
  + AeadKeyTemplates.AES128_GCM);
  // and write it to a file...
  String keysetFilename = "my_keyset.json";
  