Fixes in class declaration

Fix typo in syntax block.

Add that class name cannot be 'self', 'parent' or 'static'.
I see your idea, which is good IMO, but it cannot be *qualified-name* here since *qualified-name* includes an optional namespace name as a prefix (see [09-lexical-structure.md](https://github.com/php/php-langspec/blob/master/spec/09-lexical-structure.md#user-content-names)). It should only be *name* as before.