Correct section order in flycheck.texi

To avoid these verbose warning messages.

$ makeinfo flycheck.texi
/home/hyungchan/EmacsDot2/el-get/flycheck/doc//flycheck.texi:1683: Next field of node `Extending checkers' not pointed to (perhaps incorrect sectioning?).
/home/hyungchan/EmacsDot2/el-get/flycheck/doc//flycheck.texi:1700: This node (Examples) has the bad Prev.
/home/hyungchan/EmacsDot2/el-get/flycheck/doc//flycheck.texi:1683: Prev field of node `Extending checkers' not pointed to.
/home/hyungchan/EmacsDot2/el-get/flycheck/doc//flycheck.texi:1667: This node (Option filters) has the bad Next.
makeinfo: Removing output file `/home/hyungchan/EmacsDot2/el-get/flycheck/doc/flycheck.info' due to errors; use --force to preserve.