fix missing backslash in `vttestserver` docs

[frouioui]

Fixes https://github.com/vitessio/vitess/issues/15231

There is a missing backslash in the example to run vttestserver on Docker, which leads to the following error:

invalid argument "" for "--port" flag: strconv.ParseInt: parsing "": invalid syntax

Additionally, the path for the data volume is changed from /vt/vtdataroot/vitess to /vt/vtdataroot. The ./vitess directory does not exist and is irrelevant, we should attach the volume to the root of vtdataroot instead.

The --data_dir flag has also changed to point to the root of vtdataroot too.

[netlify[bot]]

✅ Deploy Preview for vitess ready!

Name	Link
🔨 Latest commit	174060064ddca8230c0185d03eadf9274c9d2462
🔍 Latest deploy log	https://app.netlify.com/sites/vitess/deploys/65ccfd8bb5c33400099b4ed6
😎 Deploy Preview	https://deploy-preview-1689--vitess.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.