Replace 'is comprised of' with 'comprises'

Replace 'is comprised of' with 'comprises'
because the 'is comprised of' is disputed use.

Change-Id: If66d2e4583b00102d52635457f3c3f8c2adee1be
Closes-Bug: #1831309

Author: natsumetakashi

doc/source/user/architecture.rst

@@ -18,7 +18,7 @@
 Nova System Architecture
 ========================
 
-Nova is comprised of multiple server processes, each performing different
+Nova comprises multiple server processes, each performing different
 functions. The user-facing interface is a REST API, while internally Nova
 components communicate via an RPC message passing mechanism.