Update instructions for Italian translation to adapt them to new generic langcode.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash-docs/trunk@23665 57a11ea4-9604-0410-9ed3-97b8803252fd
Update Italian translation of guide.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash-docs/trunk@23666 57a11ea4-9604-0410-9ed3-97b8803252fd