showmap: document in help memory is in KB

I'm not sure of a better place to put this in the output, but
because it's reading from smaps, it should probably be
documented elsewhere. Otherwise, you have to know.

Bug: N/A
Test: N/A
Change-Id: Ica7f8c9b6b528b37f9c2c581cafddabdaa46d66b
1 file changed