Fix CHANGELOG formatting

The release notes were missing on the GitHub Releases page for the
client for several recent releases. This happened because of incorrect
heading levels used for subsections within a release in the CHANGELOG
file.

CHANGELOG.md

@@ -6,20 +6,20 @@ Entries in this change log follow the format suggested at http://keepachangelog.
 
 ## [0.58.0] - 2017-02-21
 
-## Fixed
+### Fixed
 
 - Highlights not saving on pages with many highlights and annotations
   ([#234](https://github.com/hypothesis/client/pull/234)).
 
 ## [0.57.0] - 2017-02-17
 
-## Added
+### Added
 
 - Add a boot script to the client which replaces the `/embed.js` script served
   by the Hypothesis service
   ([#215](https://github.com/hypothesis/client/pull/215)).
 
-## Changed
+### Changed
 
 - Correct CC0 terminology
   ([#222](https://github.com/hypothesis/client/pull/222)).
@@ -32,12 +32,12 @@ Entries in this change log follow the format suggested at http://keepachangelog.
 
 ## [0.56.0] - 2017-02-14
 
-## Changed
+### Changed
 
 - Improve math rendering by updating KaTeX to v0.7.1
   ([#224](https://github.com/hypothesis/client/pull/224)).
 
-## Fixed
+### Fixed
 
 - Wait for annotations to be fetched before displaying annotation counts in the
   page ([#225](https://github.com/hypothesis/client/pull/225)).