Fix E2E browser CI flakiness by reducing example number of browser

It seems like on MacOS running 3 browsers is way too much and it
regularly fails.

Given that the failure is due to threshold being blown over as the CI
just can't load pages fast enough, I see no point for this to keep being
flaky.

The example is about multiple scenarios not about multiple browsers in
scenarios as well.

examples/browser/multiple-scenario.js

@@ -5,7 +5,7 @@ export const options = {
     messages: {
       executor: 'constant-vus',
       exec: 'messages',
-      vus: 2,
+      vus: 1,
       duration: '2s',
       options: {
         browser: {
@@ -16,8 +16,8 @@ export const options = {
     news: {
       executor: 'per-vu-iterations',
       exec: 'news',
-      vus: 2,
-      iterations: 4,
+      vus: 1,
+      iterations: 2,
       maxDuration: '5s',
       options: {
         browser: {