docs: update wit.md according current parser impl

[7sDream]

Updates:

• delete [ in operators since we do not have this token
• delete _ and A-Z from identifier since we force kebab case now
• add a sentence to make it clear that id is short for identifier
• add unit in ty
• remove _ in expected since we do not support this grammer anymore

In current parse impl, ty has a 'handle' id variant but behavor as same as id for resource type, it seems handle is deprecated?
I just ignore it for now.

[alexcrichton]

Looks good to me, thanks!