free: used miscalculation

[Krysztal112233]

Here's procps free's outputs

               total        used        free      shared  buff/cache   available
Mem:        32273048     9797636    15312152      200600     7826228    22475412
Swap:              0           0           0

Here's uutils free's outputs

               total        used        free      shared  buff/cache   available
Mem:        32273048    16985112    15287936      199512     7825372    22451428
Swap:              0           0           0

[Krysztal112233]

Infomation from man free

used
Used or unavailable memory (calculated as total - available)

For used memory, it should be mem_info.total - mem_info.available not mem_info.total - mem_info.free

I'll fix this issues in #35 .

[Krysztal112233]

Fixed.