Remove mentions of validate_spec from docs

[fsackur]

Hi! New user here.

I just spent an hour figuring out where validate_spec has got to, before I came across #308

It's mentioned in a code snippet in https://github.com/marshmallow-code/apispec/blob/dev/docs/special_topics.rst and maybe other places as well. It seems that should be removed.

I'm happy to PR, but do we want to just drop all mention from the docs? I would need some guidance.

I do want to validate my spec, and in #308 there was mention of openapi-spec-validator. What would be appropriate to document?

[sloria]

good catch! the docs mention is definitely an oversight. that should be replaced with direct usage of openapi-spec-validator.

PR welcome!