doc: file: fix \parm syntax and \ref dma_alignment
doc: file: fix \parm syntax and \ref dma_alignment

Resolve following warnings:
include/seastar/core/file.hh:349: warning: Found unknown command '\parm'
include/seastar/core/file.hh:292: warning: unable to resolve reference to 'dma_alignment' for \ref command
include/seastar/core/file.hh:303: warning: unable to resolve reference to 'dma_alignment' for \ref command
include/seastar/core/file.hh:318: warning: unable to resolve reference to 'dma_alignment' for \ref command

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,dreamsxin/seastar,syuu1228/seastar