[YAML Exporter] fix starred spelling (#907)

* fix starred spelling
c.f. #835

features/exporting.feature

@@ -129,7 +129,7 @@ Feature: Exporting a Journal
         """
         title: I have an @idea:
         date: 2013-04-09 15:39
-        stared: False
+        starred: False
         tags: idea, journal
 
         (1) write a command line @journal software

jrnl/plugins/yaml_exporter.py

@@ -104,10 +104,10 @@ class YAMLExporter(TextExporter):
         # source directory is  entry.journal.config['journal']
         # output directory is...?
 
-        return "title: {title}\ndate: {date}\nstared: {stared}\ntags: {tags}\n{dayone} {body} {space}".format(
+        return "title: {title}\ndate: {date}\nstarred: {starred}\ntags: {tags}\n{dayone} {body} {space}".format(
             date=date_str,
             title=entry.title,
-            stared=entry.starred,
+            starred=entry.starred,
             tags=", ".join([tag[1:] for tag in entry.tags]),
             dayone=dayone_attributes,
             body=newbody,