fix: calculate correct part of the xy plane (rust-in-action#29)

The wrong part of the parameter space was specified in the book's examples. 
The commit adjusts the parameters to better match the escape time algorithm[1], 

[1] https://en.wikipedia.org/wiki/Plotting_algorithms_for_the_Mandelbrot_set

ch2/ch2-mandelbrot/src/main.rs

@@ -70,8 +70,9 @@ fn render_mandelbrot(escape_vals: Vec<Vec<usize>>) {
 }
 
 fn main() {
-  let mandelbrot = calculate_mandelbrot(1000, 2.0, 1.0, -1.0,
+
+  let mandelbrot = calculate_mandelbrot(1000, -2.0, 1.0, -1.0,
                                         1.0, 100, 24);
 
   render_mandelbrot(mandelbrot);
-}
+}