Tweak based on support feedback. (#1116)

This one probably isn't worth retranslation any time soon, so I'll
choose to retain the existing text when I update the content in Crowdin.

Author: microbit-matt-hillsdon

lang/ui.en.json

@@ -472,7 +472,7 @@
     "description": "Header for name field"
   },
   "name-used-when": {
-    "defaultMessage": "The name is used when you save a hex file.",
+    "defaultMessage": "The name is used when you save a file.",
     "description": "Text under project name field"
   },
   "new-file-hint": {

src/messages/ui.en.json

@@ -1024,7 +1024,7 @@
   "name-used-when": [
     {
       "type": 0,
-      "value": "The name is used when you save a hex file."
+      "value": "The name is used when you save a file."
     }
   ],
   "new-file-hint": [