Sync changes from upstream repository

Author: Hubot

content/changes/2016-06-22-issue-locking-api-is-now-official.md

@@ -5,7 +5,7 @@ author_name: davidcelis
 
 We're making the [Issue locking and unlocking APIs][issue-locking-blog-post] part of the official GitHub API.
 
-During the preview period you needed to provide the `application/vnd.github.the-key-preview+sha` preview media type in the `Accept` header to opt-in to the new endpoints. Now that the preview period has ended, you no longer need to specify this custom [media type][custom-media-types].
+During the preview period you needed to provide the `application/vnd.github.the-key-preview+json` preview media type in the `Accept` header to opt-in to the new endpoints. Now that the preview period has ended, you no longer need to specify this custom [media type][custom-media-types].
 
 If you have any questions or feedback, please [get in touch with us][contact]!