Update architecture.rst:Calculating PG IDs

To Calculate PG ID, if I didn't get it wrong, CRUSH calculates the hash modulo 
the number of PGs instead of OSDs, according to osd/osd_types.cc:963 
ceph_stable_mod(pg.ps(), pg_num, pg_num_mask).

Signed-off-by: Kai Zhang <zakir.exe@gmail.com>
Merge pull request #1855 from kazhang/patch-1

doc: Update architecture.rst:Calculating PG IDs