fix: update ansible role names

- pleroma-postgres renamed to pleroma-database.
- pleroma-nginx renamed to pleroma-proxy.
- site.yml in examples is updated to reflect the change.

resolves dananglin/pleroma-ansible-playbook#12

examples/site.yml

@@ -11,9 +11,9 @@
   become: yes
   become_method: sudo
   roles:
-  - pleroma-postgres
+  - pleroma-database
   tags:
-  - pleroma-postgres
+  - pleroma-database
 
 - name: Installing and configuring the Pleroma backend.
   hosts: pleroma_backend
@@ -29,6 +29,6 @@
   become: yes
   become_method: sudo
   roles:
-  - pleroma-nginx
+  - pleroma-proxy
   tags:
-  - pleroma-nginx
+  - pleroma-proxy