Update example code in README.md (swiftwasm#100)

It was my mistake to convert the example code to the new style without checking it in a real app, turns out there were a few errors in the converted version.

Calling `alert` on `JSObject.global` still requires force unwrapping. Also, dynamic properties can't be updated on `JSValue` constants, so need to convert `divElement` to a `var` binding.

Author: MaxDesiatov

README.md

@@ -34,7 +34,7 @@ import JavaScriptKit
 
 let document = JSObject.global.document
 
-let divElement = document.createElement("div")
+var divElement = document.createElement("div")
 divElement.innerText = "Hello, world"
 _ = document.body.appendChild(divElement)
 
@@ -50,7 +50,7 @@ struct Pet: Codable {
 let jsPet = JSObject.global.pet
 let swiftPet: Pet = try JSValueDecoder().decode(from: jsPet)
 
-JSObject.global.alert("Swift is running in the browser!")
+JSObject.global.alert!("Swift is running in the browser!")
 ```
 
 ### Usage in a browser application