Fix room `type` typo in mailer (#17336)

Correct event content field is `EventContentFields.ROOM_TYPE` (`type`) ✅ , not `room_type` ❌

Spec: https://spec.matrix.org/v1.10/client-server-api/#mroomcreate

changelog.d/17336.bugfix

@@ -0,0 +1 @@
+Fix email notification subject when invited to a space.

synapse/push/mailer.py

@@ -28,7 +28,7 @@ import jinja2
 from markupsafe import Markup
 from prometheus_client import Counter
 
-from synapse.api.constants import EventTypes, Membership, RoomTypes
+from synapse.api.constants import EventContentFields, EventTypes, Membership, RoomTypes
 from synapse.api.errors import StoreError
 from synapse.config.emailconfig import EmailSubjectConfig
 from synapse.events import EventBase
@@ -716,7 +716,8 @@ class Mailer:
                 )
                 if (
                     create_event
-                    and create_event.content.get("room_type") == RoomTypes.SPACE
+                    and create_event.content.get(EventContentFields.ROOM_TYPE)
+                    == RoomTypes.SPACE
                 ):
                     return self.email_subjects.invite_from_person_to_space % {
                         "person": inviter_name,