[tools/argdist.py] Fix a wrong place of help info (iovisor#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 <[email protected]>

tools/argdist.py

@@ -559,7 +559,7 @@ class Tool(object):
 argdist -C 'r::__vfs_read():u32:$PID:$latency > 100000'
  Print frequency of reads by process where the latency was >0.1ms
 
-argdist -H 'r::__vfs_read(void *file, void *buf, size_t count):size_t
+argdist -H 'r::__vfs_read(void *file, void *buf, size_t count):size_t:
  $entry(count):$latency > 1000000'
  Print a histogram of read sizes that were longer than 1ms