Update example to use const (#48262)

* update example

* add a new example

Author: meaghanlewis

docs/csharp/misc/cs0131.md

@@ -33,14 +33,34 @@ public class MyClass
     }  
     public static void Main() { }  
 }  
-```  
+```
+
+## Example 2
+
+ The following sample generates CS0131 when assigning to a constant field.  
 
-## Example 2  
+```csharp  
+// CS0131b.cs  
+public class B  
+{
+    public static int Main()
+    {
+        const int j = 0;
+        j = 1; // CS0131  
+        // try the following lines instead
+        // int j = 0; 
+        // j = 1;
+        return j;
+    }  
+} 
+```
+
+## Example 3
 
  This error can also occur if you attempt to perform arithmetic operations on the left hand side of an assignment operator, as in the following example.  
 
 ```csharp  
-// CS0131b.cs  
+// CS0131c.cs  
 public class C  
 {  
     public static int Main()