debian/control: fix recommends for icedove-l10n-sr

As pointed out by Jonas Smedegaard the field Recommends for
icedove-l10n-sr was wrong. Corrected as suggested by Jonas.

Closes: #767635
cherry picked from master branch (ba9e08f79160fe8f05f954c2051fd0c1fdff70ff)
document changes and release 1:34.0~b1-1