Fix two trivial docs errors in architecture.rst
Fix two trivial docs errors in architecture.rst

1 Correct a grammar about not only...but also...
2 Now, there are four storage drivers rather than three in gnocchi.

Change-Id: I8744adda401e7f92dc3688c51d28f958e9d85b08

gnocchixyz/gnocchi,sileht/gnocchi,leandroreox/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi,sileht/gnocchi