Typos in frontend syntax documentation

[itamarst]

In https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/syntax.md, there are some references to tags that are wrong:

• docker/dockerfile:v1.2 (notice the extra "V")
• docker/dockerfile:experimental

They should be replaced with references to docker/dockerfile:1.2 I believe.

[nidomiro]

Thank you :)
After about 2h of trial and error I encountered your issue and saw that I just had to remove the "v"

[tiborvass]

Sorry about that, thanks for catching!