use consistent capitalization for subject lines

config/locales/devise.en.yml

@@ -24,7 +24,7 @@ en:
       unlock_instructions:
         subject: "Mastodon: Unlock instructions"
       password_change:
-        subject: "Mastodon: Password Changed"
+        subject: "Mastodon: Password changed"
     omniauth_callbacks:
       failure: "Could not authenticate you from %{kind} because \"%{reason}\"."
       success: "Successfully authenticated from %{kind} account."