(core) Fix flaky DocTutorial test

Summary:
The substring for the anchor link row id was missing a leading ".", which was
a problem because tutorial fork ids would sometimes contain the substring.

Test Plan: N/A

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3853

test/nbrowser/DocTutorial.ts

@@ -234,7 +234,7 @@ describe('DocTutorial', function () {
     it('does not play an easter egg when opening an anchor link encoded with rr', async function() {
       await gu.getCell({rowNum: 1, col: 0}).click();
       const link = await gu.getAnchor();
-      const easterEggLink = link.replace('r1', 'rr1');
+      const easterEggLink = link.replace('.r1', '.rr1');
       await driver.get(easterEggLink);
       await gu.waitForAnchor();
       assert.isFalse(await driver.find('.test-behavioral-prompt').isPresent());