message: Fix the doc comment for get_pixel_mut
  The doc comment for get_pixel_mut describes
  the put_pixel method instead of the get_pixel_mut
  method. This commit copies the get_pixel_mut doc
  comment from buffer.rs and replaces the doc comment
  in image.rs.
diff: @@ -433,7 +433,7 @@ pub trait GenericImage: GenericImageView {
  /// indirections and it eases the use of nested SubImages.
  type InnerImage: GenericImage<Pixel = Self::Pixel>;
  - /// Puts a pixel at location (x, y)
  + /// Gets a reference to the mutable pixel at location `(x, y)`
  ///
  /// # Panics
  ///
  