merge lesson2,3 in chapter1

moonbit-tour/tour/chapter1_basics/lesson1_variable/index.mbt

@@ -2,4 +2,12 @@ fn main {
   let a : Int = 10 
   let b = 20 
   println(a + b)
+
+  let mut  c = 10 
+  c = c + 1
+  println(c)
+
+  let d = 20 
+  // d = d + 1
+  println(d)
 }

moonbit-tour/tour/chapter1_basics/lesson1_variable/index.md

@@ -5,4 +5,7 @@ The `let` keyword used to define a variable.
 The type of the variable can be annotated by using a colon followed by the type. 
 It is optional, if not provided the type will be inferred from the value.
 
+Variables are immutable by default in MoonBit. You can add an extra `mut` 
+keyword to make them mutable at the local level.
 
+If you uncomment the `d = d + 1`, you will get an error.

moonbit-tour/tour/chapter1_basics/lesson3_numbers/index.mbt → moonbit-tour/tour/chapter1_basics/lesson2_numbers/index.mbt

@@ -5,6 +5,12 @@ fn main {
   let oct = 0o777
   let bin = 0b1001 
 
+  println(1 + 2)
+  println(1 - 2)
+  println(1 * 2)
+  println(5 / 2)
+  println(10 % 3)
+
   let num1 : Double = 3.14
   let num2 : Float = 3.14
 }