edit the factiruo button description

src/simplejavacalculator/UI.java

@@ -97,7 +97,7 @@ public UI() throws IOException {
       butDivide = new JButton("/");      
       butEqual = new JButton("=");      
       butSquareRoot = new JButton("sqrt");  
-      butFactorial = new JButton("!");      
+      butFactorial = new JButton("Factorial!");
       butSquare = new JButton("x*x");      
       butOneDividedBy = new JButton("1/x");      
       butCos = new JButton("Cos");