Fix config expression in cds-server.md

[mika-espoo]

Instructions are missing env property from the chain and thus have no effect. This patch fixes the documentation from this part.

[smahati]

adding @johannes-vogel and @schwma to review this PR

[chgeo]

Well, cds.env.server.cors = true could mean

• Programmatic config with cds.env.server.cors = true (the literal expression, but used rarely)
• Config through package.json etc. with cds: {server: { cors: true }} with the shorthand notation cds.server.cors:true (much more common)

Looking at our docs, it's really a mixed bag with one or the other of the above options. Whatever we currently do, there are users that will get confused.

This is why I plan to add a little 'config inspector' that would appear as a popover for such a config key showing the user all config options.

For now, the suggested fix is ok, I'd say.