Fix `sr@latin.po` place

The sr@latin.po was placed inside ./desktop-docs/gpl/sr/, when should be in ./desktop-docs/gpl/sr@latin/. The wrongly placed translation file causes the following:

1- it is not recognized by Damned Lies (therefore, this contribution can't be seen in the translation management system); and

2- it is not used to generate the intended documentation.

This issue was reported by Jean-Baptiste Holcroft in https://mail.gnome.org/archives/gnome-i18n/2019-April/msg00040.html

This commit moves the contribution to the proper place: sr@latin/sr@latin.po