Use ~s for os:getpid().

os:getpid() returns a string. So using ~p leaves double quotes around
the value.

Change-Id: Ie2a509aa2709e611cd801e37b4987886662112ef
Reviewed-on: http://review.couchbase.org/43496
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>