Fix formatting in breaking changes doc

Signed-off-by: Florian Maier <contact@marsmenschen.com>

docs/breaking_changes.md

@@ -33,7 +33,8 @@ breaking change. Images pushed by Engine 1.10 to a Registry 2.3 cannot be
 pulled by digest by older Engine versions. A `docker pull` that encounters this
 situation returns the following error:
 
-``` Error response from daemon: unsupported schema version 2 for tag TAGNAME
+```
+ Error response from daemon: unsupported schema version 2 for tag TAGNAME
 ```
 
 Docker Content Trust heavily relies on pull by digest. As a result, images