vmstat: fix format

[Bluemangoo]

fix #379

now putput is like:

$ cargo run -q vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- -------cpu-------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st gu
 1  0      0 6833000 335140 3992124  0    0  6168  1252 2863 8799  6  1 93  0  0  0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (047df29) to head (2d3f096).
Report is 18 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #383   +/-   ##
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