Update heroku.md

Fix typo.

Author: rtwomey

docs/heroku.md

@@ -15,7 +15,7 @@ When you see `rake assets:precompile` during deployment. Heroku is actually runn
 
 This means the *RAILS_ENV* you have set via *heroku:config* is not used.
 
-**Workaround:** you could have just one S3 bucket dedicated to assets and ensure to set keep the existing remote files
+**Workaround:** you could have just one S3 bucket dedicated to assets and configure `AssetSync` to not delete existing files:
 
     AssetSync.configure do |config|
       ...
@@ -33,4 +33,4 @@ Currently when heroku runs `rake assets:precompile` during deployment. It does n
       config.aws_access_key_id = 'xxx'
       config.aws_secret_access_key = 'xxx'
       config.fog_directory = 'mybucket'
-    end
+    end