Fix: use let to prevent reassignment error for sandbox result (#1067)

In the sandbox/sandbox example, a variable `result` is created as a const, but it's attempted to be reassigned later in the code.  This changeset just changes the `result` variable to be a reassignable variable instead.

api-samples/sandbox/sandbox/sandbox.html

@@ -57,8 +57,8 @@ <h1>Welcome back</h1>
       // Set up message event handler:
       window.addEventListener('message', function (event) {
         const command = event.data.command;
-        const template = templates[event.data.templateName],
-          result = 'invalid request';
+        const template = templates[event.data.templateName];
+        let result = 'invalid request';
 
         // if we don't know the templateName requested, return an error message
         if (template) {