Fix typo in Flatpak English description.

[J0anJosep]

Just change isntalled with installed.

Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[CommandMC]

Thank you for your contributions!
The Flatpak-related files in this repo aren't really that important, as they're only used when installing a testing build from source. You probably want to submit your PRs to flathub/com.heroicgameslauncher.hgl instead.

[J0anJosep]

Thanks!