message: doc: Update comments of PnmEncoder constructor about type name
diff: @@ -79,7 +79,7 @@ enum TupleEncoding<'a> {
  }
  impl<W: Write> PnmEncoder<W> {
  - /// Create new PNMEncoder from the `writer`.
  + /// Create new PnmEncoder from the `writer`.
  ///
  /// The encoded images will have some `pnm` format. If more control over the image type is
  /// required, use either one of `with_subtype` or `with_header`. For more information on the
  