[tools/argdist.py] Fix a wrong place of help info (#1939)

argdist -H 'r::__vfs_read(void *file, void *buf, size_t count):size_t
$entry(count):$latency > 1000000'
is a wrong example which cann't excute success, because lack of ":" and cann't split field correctly.

So, the right command is:

argdist -H 'r::__vfs_read(void *file, void *buf, size_t count):size_t:
$entry(count):$latency > 1000000'

Signed-off-by: Ahao Mu <muahao@linux.alibaba.com>