fix(e2e): update test of viewport to match latest example stories

Example stories used in CLI have been updated recently however the E2E tests were not changed accordingly.

cypress/generated/addon-viewport.spec.ts

@@ -4,7 +4,7 @@ describe('addon-viewport', () => {
  });
 
  it('should have viewport button in the toolbar', () => {
- cy.navigateToStory('button', 'Text');
+ cy.navigateToStory('example-button', 'Primary');
 
  // Click on viewport button and select small mobile
  cy.get('[title="Change the size of the preview"]').click();