Fix documentation test in graphics module

src/graphics.rs

@@ -85,7 +85,7 @@ pub struct Cell {
 /// ```rust,no_run
 /// use arrayfire::{histogram, load_image, Window};
 /// let mut wnd = Window::new(1280, 720, String::from("Image Histogram"));
-/// let img = load_image("Path to image".to_string(), true/*If color image, 'false' otherwise*/);
+/// let img = load_image::<f32>("Path to image".to_string(), true/*If color image, 'false' otherwise*/);
 /// let hst = histogram(&img, 256, 0 as f64, 255 as f64);
 ///
 /// loop {