Change name of flag to match unstable prefix

Use "im.nheko" instead of "org.matrix"

synapse/rest/client/versions.py

@@ -133,7 +133,7 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
                     # Supports receiving private read receipts as per MSC2285
                     "org.matrix.msc2285.stable": True,  # TODO: Remove when MSC2285 becomes a part of the spec
                     # Supports room previews as per MSC3266
-                    "org.matrix.msc3266": self.config.experimental.msc3266_enabled,
+                    "im.nheko.msc3266": self.config.experimental.msc3266_enabled,
                     # Supports filtering of /publicRooms by room type as per MSC3827
                     "org.matrix.msc3827.stable": True,
                     # Adds support for thread relations, per MSC3440.