finish renaming 'Scratch Desktop' to 'Scratch'

- adjust macOS installation instructions to mention "Scratch" instead of
  "Scratch Desktop"
- edit the DMG screenshot to fix the app name, DMG title, and DMG icon

src/components/install-scratch/install-scratch.jsx

@@ -114,7 +114,7 @@ const InstallScratch = ({
                         <span className="step-description">
                             {currentOS === OS_ENUM.WINDOWS ?
                                 <FormattedMessage id="download.winMoveToApplications" /> :
-                                <FormattedMessage id="download.macMoveToApplications" />
+                                <FormattedMessage id="download.macMoveAppToApplications" />
                             }
                         </span>
 

src/views/download/l10n.json

@@ -18,6 +18,7 @@
   "download.scratch2Desktop" : "Scratch 2.0 Offline Editor",
   "download.cannotAccessMacStore" : "What if I can't access the Mac App Store?",
   "download.cannotAccessWindowsStore" : "What if I can't access the Microsoft Store?",
+  "download.macMoveAppToApplications": "Open the .dmg file. Move Scratch into Applications.",
   "download.macMoveToApplications" :  "Open the .dmg file. Move Scratch Desktop into Applications.",
   "download.winMoveToApplications" : "Run the .exe file.",
   "download.doIHaveToDownload" : "Do I have to download an app to use Scratch?",