Documentation update for DROPBOX_REMOTE_PATH environment variable (#473)

* Update set-up-dropbox.md

Updated text for explanation of DROPBOX_REMOTE_PATH environment variable usage accordingly to how the Dropbox App was scoped during creation.

* Update set-up-dropbox.md

Minor style edit

* Update index.md

Replaced /Apps/my-test-app/some_subdir with /somedir in example value for DROPBOX_REMOTE_PATH environment variable under "Backing up to Dropbox" section, to align with updates done to related "how-to" text

docs/recipes/index.md

@@ -190,7 +190,7 @@ services:
       DROPBOX_REFRESH_TOKEN: REFRESH_KEY  # replace
       DROPBOX_APP_KEY: APP_KEY  # replace
       DROPBOX_APP_SECRET: APP_SECRET  # replace
-      DROPBOX_REMOTE_PATH: /Apps/my-test-app/some_subdir  # replace
+      DROPBOX_REMOTE_PATH: /somedir  # replace
     volumes:
       - data:/backup/my-app-backup:ro
       - /var/run/docker.sock:/var/run/docker.sock:ro