Update README.md

fix demo code compile warning

Author: light4

README.md

@@ -226,7 +226,7 @@ extern crate plotters;
 use plotters::prelude::*;
 
 let figure = evcxr_figure((640, 480), |root| {
-    root.fill(&WHITE);
+    root.fill(&WHITE)?;
     let mut chart = ChartBuilder::on(&root)
         .caption("y=x^2", ("Arial", 50).into_font())
         .margin(5)