Use @OptIn(ExperimentalContracts::class) instead of @ExperimentalContracts on createElement funs in Builders.kt

Fixed issue 11

Author: igoriakovlev

src/wasmJsMain/kotlin/kotlinx/dom/Builders.kt

@@ -14,7 +14,7 @@ import kotlin.contracts.*
  * The element is initialized with the specified [init] function.
  */
 @SinceKotlin("1.4")
-@ExperimentalContracts
+@OptIn(ExperimentalContracts::class)
 public fun Document.createElement(name: String, init: Element.() -> Unit): Element {
     contract { callsInPlace(init, InvocationKind.EXACTLY_ONCE) }
     return createElement(name).apply(init)
@@ -26,7 +26,7 @@ public fun Document.createElement(name: String, init: Element.() -> Unit): Eleme
  * The element is initialized with the specified [init] function.
  */
 @SinceKotlin("1.4")
-@ExperimentalContracts
+@OptIn(ExperimentalContracts::class)
 public fun Element.appendElement(name: String, init: Element.() -> Unit): Element {
     contract { callsInPlace(init, InvocationKind.EXACTLY_ONCE) }
     return ownerDocument!!.createElement(name, init).also { appendChild(it) }