Fix backend example

Author: A-K-O-R-A

README.md

@@ -327,7 +327,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     // Create a 800*600 bitmap and start drawing
     let mut backend = BitMapBackend::new("plotters-doc-data/1.png", (300, 200));
     // And if we want SVG backend
-    // let backend = SVGBackend::new("output.svg", (800, 600));
+    // let mut backend = SVGBackend::new("output.svg", (800, 600));
     backend.draw_rect((50, 50), (200, 150), &RED, true)?;
     backend.present()?;
     Ok(())