Fix `[x]` in simple table markup

Summary:
This `*` should be a `+` since we want to match //something//.

Fixes `[x]` in a simple table, for any `x` which is not a link.

Test Plan: Added unit test, made it pass.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5934