Fix: Replace Int: x -> x: Int (#1620)

Declaring variables should use `var x: Int`.

explorer/README.md

@@ -224,8 +224,8 @@ The first time increments `x` to `1` and the second time increments `x` to `2`,
 so the expected result of this program is `2`.
 
 ```carbon
-fn Main() -> Int {
-  var Int: x = 0;
+fn Main() -> i32 {
+  var x: i32 = 0;
   __continuation k {
     x = x + 1;
     __await;