Fix "abou" typo of "about" in SearchEngine API methods

Summary: See https://discourse.phabricator-community.org/t/minor-typo-abou-in-conduit-help/2498/2.

Test Plan: Read carefully.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20280