Fix a typo in the card activity

Fixes #366

i18n/en.i18n.json

@@ -14,7 +14,7 @@
     "activity-on": "on %s",
     "activity-removed": "removed %s from %s",
     "activity-sent": "sent %s to %s",
-    "activity-unjoined": "unjoinded %s",
+    "activity-unjoined": "unjoined %s",
     "add": "Add",
     "add-attachment": "Add an attachment",
     "add-board": "Add a new board",