Update "property" type parameter for "optionalMemberExpression"

Currently the section for [optionalMemberExpression ](https://babeljs.io/docs/en/babel-types#optionalmemberexpression) is saying that `property` parameter could be `any`, but it isn't true anymore, as well [could be noticed on the code](https://github.com/babel/babel/blob/e9c1bce50f590c484e9ba7cc0b19caa88e66659d/packages/babel-types/src/definitions/experimental.js#L87-L88).

I think that it should be update to: if `computed` then `Expression` else `Identifier` (required)`