Update path to file templates in README

It looks like on October 1st https://github.com/slackhq/SlackTextViewController/commit/f729fd9a1ea4ddd6f3e600dbb49c5dd36cdf2969 was the initial addition of the Xcode templates info to the README, and on October 7th https://github.com/slackhq/SlackTextViewController/commit/05d8a69e78042c8b33a186f66493d7cee90b14ff renamed the templates folder without fixing the accompanying reference.