fix spelling error 🐛

I think lots of people are still reading these codes like me,  so fix the spelling mistake is necessary.

blogpost/api.py

@@ -20,7 +20,7 @@ def has_permission(self, request, view):
         return False
 
 
-class BlogpsotSerializer(serializers.HyperlinkedModelSerializer):
+class BlogpostSerializer(serializers.HyperlinkedModelSerializer):
     class Meta:
         model = Blogpost
         fields = ('title', 'author', 'body', 'slug', 'id')