quick fix: Isolation mode feature flag name udpate

Author: BillCarsonFr

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -125,7 +125,7 @@ enum class FeatureFlags(
     ),
     OnlySignedDeviceIsolationMode(
         key = "feature.onlySignedDeviceIsolationMode",
-        title = "Exclude not secure devices when sending/receiving messages",
+        title = "Exclude insecure devices when sending/receiving messages",
         description = "This setting controls how end-to-end encryption (E2E) keys are shared." +
             " Enabling it will prevent the inclusion of devices that have not been explicitly verified by their owners." +
             " You'll have to stop and re-open the app manually for that setting to take effect.",