core/sharded.hh: tweak on comment for better readability
core/sharded.hh: tweak on comment for better readability

just some minor tweaks, easier to digest this way.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

Closes #1217

scylladb/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar