Fix async step callback (#4869)

Author: emilioalvap

docs/en/observability/synthetics-create-test.asciidoc

@@ -106,10 +106,10 @@ A basic two-step journey would look like this:
 [source,js]
 ----
 journey('Journey name', ({ page, browser, client, params, request }) => {
-    step('Step 1 name', () => {
+    step('Step 1 name', async () => {
       // Do something here
     });
-    step('Step 2 name', () => {
+    step('Step 2 name', async () => {
       // Do something else here
     });
 });
@@ -120,7 +120,7 @@ For example, a basic first step might load a web page:
 
 [source,js]
 ----
-step('Load the demo page', () => {
+step('Load the demo page', async () => {
   await page.goto('https://elastic.github.io/synthetics-demo/'); <1>
 });
 ----