[IMP] project_issue : correct model name in onchange method

There is no model project.issueS, making the onchange select an inexistant model.
Fixes #3418

addons/project_issue/project_issue.py

@@ -564,7 +564,7 @@ class project_project(osv.Model):
         if use_tasks and not use_issues:
             values['alias_model'] = 'project.task'
         elif not use_tasks and use_issues:
-            values['alias_model'] = 'project.issues'
+            values['alias_model'] = 'project.issue'
         return {'value': values}
 
     def create(self, cr, uid, vals, context=None):