docs: Document "flatpak update --appstream" better

The manpage description for flatpak-update's --appstream option says it
updates appstream data for the remote, but it's unclear that the remote
should be passed on the command line instead of a ref. This commit
updates the manpage to document that form of the command.

Closes: #1177
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f