Changed string "Untitles" to "Untitled"

frontend/app_flowy/assets/translations/en.json

@@ -1,5 +1,5 @@
 {
-  "appName": "Appflowy",
+  "appName": "AppFlowy",
   "defaultUsername": "Me",
   "welcomeText": "Welcome to @:appName",
   "githubStarText": "Star on GitHub",
@@ -69,7 +69,7 @@
   },
   "menuAppHeader": {
     "addPageTooltip": "Quickly add a page inside",
-    "defaultNewPageName": "Untitles",
+    "defaultNewPageName": "Untitled",
     "renameDialog": "Rename"
   },
   "toolbar": {

frontend/app_flowy/lib/generated/codegen_loader.g.dart

@@ -85,7 +85,7 @@ class CodegenLoader extends AssetLoader{
   },
   "menuAppHeader": {
     "addPageTooltip": "Quickly add a page inside",
-    "defaultNewPageName": "Untitles",
+    "defaultNewPageName": "Untitled",
     "renameDialog": "Rename"
   },
   "toolbar": {