fix a character

resource/locales/en-US/translation.json

@@ -369,7 +369,7 @@
     "image": "Image",
     "alt_text": "Alt Text",
     "insert_image": "inserts an image",
-    "open_sandbox": "Open sandbox"
+    "open_sandbox": "Open Sandbox"
   },
   "hackmd": {
     "not_set_up": "HackMD is not set up.",