Change the about.instance_actor_flash to be single-line (#14200)

Some translations of that string are single-line, which somehow seems to make
Crowdin issue a blank newline at the end of those translations.

This, in turns, leads to different results when running “i18n-tasks normalize”
depending on the version of libyaml installed, making the CI fail if it
runs a different version than whoever ran “i18n-tasks normalize”.

Since there is no real reason for that source string to be multi-line (it is
only displayed in HTML, without replacing newlines by <br/> tags),
attempt to fix Crowdin export by making the source string single-line.