Delete whitespace between brackets.  Because Github Flavored Markdown
was changed [1].

[1]: https://githubengineering.com/a-formal-spec-for-github-markdown/

Test Plan: build

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3534