switch.ftl: missed Facebook as placeable.

[peiying2]

@alexgibson when tou migrate the next batch of files, let's update the file to include Facebook:

www-l10n/en/firefox/switch.ftl

Line 20 in 843002b

switch-share-to-facebook = Share to Facebook

[flodolo]

@peiying2
You can't update existing strings without new IDs, especially when doing changes that will cause a warning to show up for all locales.

I'm starting to think this is what caused https://bugzilla.mozilla.org/show_bug.cgi?id=1648377

[peiying2]

mozilla/bedrock#9056