Bug 941993 - Change a debug-only pluralized number of seconds string.

It was raised in bug 941993 that we would display "1 seconds" if a value of 1
was used.  This wouldn't happen, but I left the bug open because it's arguably
silly to have the incorrect value.

I am not updating the l10n string since this is a debug only pluralized string.
No user will ever see this string unless they use the secret debug menu to set
their periodic sync value to a ridiculously short value (20 seconds or 60
seconds.)  And in that case the existing localized strings are already correct.

I'm just doing this to close out the bug.
Merge pull request #25848 from asutherland/email-nit-seconds-l10n

Bug 941993 - Change a debug-only pluralized number of seconds string. r=jrburke