Modify some spelling mistakes

There are some comment errors, it's modify 'Captial' to 'Capital' in
keystone file, and modify 'possition' to 'position' in openrc file, and
modify 'comming' to 'coming' in stack file, and
modify 'prefered' to 'preferred' in stackrc file.

Change-Id: I0fdd539cbfff842a4ba7fca9100b881443300f9a
diff --git a/openrc b/openrc
index 23c173c..37724c5 100644
--- a/openrc
+++ b/openrc
@@ -84,7 +84,7 @@
 # We currently recommend using the version 3 *identity api*.
 #
 
-# If you don't have a working .stackenv, this is the backup possition
+# If you don't have a working .stackenv, this is the backup position
 KEYSTONE_BACKUP=$SERVICE_PROTOCOL://$SERVICE_HOST:5000
 KEYSTONE_AUTH_URI=${KEYSTONE_AUTH_URI:-$KEYSTONE_BACKUP}