「DOM」の節のgetElementByIdのサンプルプログラムのタイポを修正 (#924)

Author: chvmvd

docs/1-trial-session/13-dom/_samples/get-element-by-id/script.js

@@ -1,2 +1,2 @@
-const element = document.getElementById("greeting");
-element.textContent = "Hello DOM";
+const greetingElement = document.getElementById("greeting");
+greetingElement.textContent = "Hello DOM";