[i18n] upgradeMsg2 uses fr for en

[JJCUBER]

Describe the bug

The secondary upgrade message in i18n/en/background.json uses French for English (which shows up as a message when upgrading the extension to newer versions). This bug seems to have been introduced here: 71e6380b66c0fb409420b5aac42f377e2cff261b

@@ -78,5 +78,5 @@
  "turnOff": "$1 has been turned off",
  "Upgrade": "Upgrade",
  "upgradeMsg": "Vimium C has been upgraded to v$1.\n",
- "upgradeMsg2": "Easier to add key mapping and text substitution rules."
+ "upgradeMsg2": "Plus facile d'ajouter un mappage de touches et d'ajouter un presse-papiers temporaire interne."
}

(I have omitted the remaining sections since they are not applicable to this kind of bug.)

[gdh1995]

Um sorry I'll fix it then.