fix typo and improve test to catch it

examples/answers-raid.yaml

@@ -41,7 +41,7 @@ Filesystem:
           - [disk index 0, part 3]
           - spare
     - obj: [raid name md1]
-      action: FORMAT
+      action: PARTITION
       data:
         fstype: ext4
         mount: /

subiquity/models/filesystem.py

@@ -693,7 +693,7 @@ class FilesystemModel(object):
                 if can_emit(obj):
                     emit(obj)
                 else:
-                    next_work.apped(obj)
+                    next_work.append(obj)
             if len(next_work) == len(work):
                 raise Exception(
                     "rendering block devices made no progress: {}".format(