gui: Add missing translation string to log viewer

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4714

gui/default/assets/lang/lang-en.json

@@ -33,6 +33,7 @@
    "Automatic upgrade now offers the choice between stable releases and release candidates.": "Automatic upgrade now offers the choice between stable releases and release candidates.",
    "Automatic upgrades": "Automatic upgrades",
    "Automatically create or share folders that this device advertises at the default path.": "Automatically create or share folders that this device advertises at the default path.",
+   "Available debug logging facilities:": "Available debug logging facilities:",
    "Be careful!": "Be careful!",
    "Bugs": "Bugs",
    "CPU Utilization": "CPU Utilization",