订正essentail class chapter

Author: subaochen

guide/code/essentials/src/cn/edu/sdut/softlab/essentials/string/StringBufferTest.java

@@ -2,7 +2,7 @@
 
 public class StringBufferTest {
     public static void main(String[] args) {
-        int loops = 20000;
+        int loops = 5000;
         System.out.println(System.currentTimeMillis());
         String s = "";
         for(int i = 0; i < loops; i++)

guide/essentail_classes.lyx

@@ -1230,7 +1230,8 @@ status open
 \begin_layout Plain Layout
 \align center
 \begin_inset Graphics
-	filename imgs/essentail-classes/number-classes.svg
+	filename imgs/essentail-classes/number-classes.eps
+	lyxscale 60
 	width 60col%
 
 \end_inset