Wrong instructions to build the extension

[anthonyvdotbe]

As was already brought up in #140, in the build instructions at https://github.com/oracle/javavscode/blob/main/BUILD.md#fetching-and-building-netbeans-code the line cd .. must be removed.

[Achal1607]

You need to clone NetBeans into the top-level javavscode directory. Since the ant targets are located in this directory, return to it after checking out the appropriate branch in NetBeans. Due to that cd.. comes in the build steps.

If you find this comment useful then we can add a new cd javavscode step in BUILD.md steps to avoid future confusion.

[anthonyvdotbe]

Thanks for your explanation, I get it now and created a PR to clarify the steps a bit.

[arvindaprameya]

Thank you for your contribution @anthonyvdotbe !!