doc: typo - Table 1-2 should refer to "status", not "statm"

Fixes a typo in proc.txt documentation. Table 1-2 should refer to
"status", not "statm"

Signed-off-by: Mulyadi Santosa <mulyadi.santosa@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>