[breaking] Comply with YARD documentation on Hash tag format

[apiology]

Fixes #961

This is technically a breaking change, but should be pretty safe - doesn't look like we ever parsed key types out of Hash<A, B>. Here's the spec as of 2018: https://github.com/castwide/solargraph/commit/5209b491a4be5ab5a8d48eaec66d397fd41fc527

See https://www.rubydoc.info/gems/yard/file/docs/Tags.md#hashes