Correct spelling: s/[sS]yncro/[sS]ynchro/

There is widespread use of words related to term synchronization
in VLC project. Also, common mistake is dropping letter H from
such words. This commit addresses this issue with exception of
not introducing letter H to word [sS]ync, thou [sS]ynch is also
correct, but really rarely used.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>