Update README.md

Author: iqiukp

README.md

@@ -137,12 +137,19 @@ Visualization.plotDecisionBoundary(SVDD, model, trainData, trainLabel);
 
 * decision boundary
 
-<img src="https://github.com/iqiukp/Support-Vector-Data-Description-SVDD/blob/master/img/contour.png" width="480"><img src="https://github.com/iqiukp/Support-Vector-Data-Description-SVDD/blob/master/img/decision-boundary.png" width="480">
+<p align="middle">
+  <img src="https://github.com/iqiukp/Support-Vector-Data-Description-SVDD/blob/master/img/contour.png" width="420">
+  <img src="https://github.com/iqiukp/Support-Vector-Data-Description-SVDD/blob/master/img/decision-boundary.png" width="420">
+</p>
 
 * testing result
 
-<img src="https://github.com/iqiukp/Support-Vector-Data-Description-SVDD/blob/master/img/distance.png" width="480">
+<p align="middle">
+  <img src="https://github.com/iqiukp/Support-Vector-Data-Description-SVDD/blob/master/img/distance.png" width="720">
+</p>
 
 * ROC curve
 
-<img src="https://github.com/iqiukp/Support-Vector-Data-Description-SVDD/blob/master/img/ROC.png" width="480">
+<p align="middle">
+  <img src="https://github.com/iqiukp/Support-Vector-Data-Description-SVDD/blob/master/img/ROC.png" width="480">
+</p>